Oh I feel for you. We went through something quite similar with our GSD almost 2 years ago. I actually posted a question about it at the time. Our vet couldn't figure out what was wrong (she had a bulging eye and swollen head on one side--though she is an inside dog and never is outside alone, and we found no evidence of ANY type of trauma having happened to her--not even a mark on her. We thought a snake bite or spider bite even, but no evidence of that either). High fever was about it, except for swelling on one side of her head, with the protruding 3rd eyelid. The vet gave us pain meds and an antibiotic (her white count was high) on the first day before the seizures started...but she only got worse (falling down) and then the seizures started. She was having seizures on day 2. We didn't realize what they were though. She didn't lay there and shake... she would just freeze up and then her head/neck would go back really stiff and farther than it should have, with her front legs out stiff in front of her. It was intermittent. They would last a minute or two, then she would stop and lay there. She seemed sad and scared. We took her back in when she couldn't walk and were basically told what you seem to have heard as well. No way to tell without an MRI ($2000). Otherwise, watch, wait, and consider euthanasia.
Well, our girl had stopped eating even. She only drank a few sips if I held a bowl up to her. She could no longer stand up and had to be carried outside to pee. She refused, even in that state, to go in the house. I felt like it would have embarrassed her to have done that. So I made my husband carry her out to the grass, and we supported her weight as she tried to crouch. She was scared, but we encouraged her and you could feel the relief in her when she let it gush out. (Sorry to be so graphic).
In the middle of the night (on about day 3 or 4?) she had a full out seizure where she was flailing and running with her legs so that she was spinning herself in a circle laying flat on the floor. My husband got up with her, and tried to calm her and keep her from hitting the furniture legs. He also was so upset by the prospects and the pain we thought she must be in that he crushed up her pain pill (we hadn't been able to force her to eat one--or anything for that matter in a day or two), and mixed it into some protein powder he had and a little water, and used a syringe to shoot it into her mouth. He slept on the sofa the rest of the night to be near her.
The next morning when I got up to for school prep, she was lying on her bed off to one side of the room and listening to the kids walking around getting ready. It was evident that she couldn't SEE them. But she would follow the sounds and smells with her attention. I started giving her more of the protein shake mix throughout the day and brought home some rotisserie chicken to offer her at dinner. The next day, she was attempting to stand on her own. By the end of that day she was walking like a drunk sailor, very slowly around the yard, and enjoying being outside.
We had house guests arrive that Thursday night. She started seizing again. Friday morning I took her to another vet for a 2nd opinion. Got a full discussion of what they could and could not do and check for and what each thing would cost. We agreed to give it a try. For around $600 they admitted her, gave her on diazapam, put in an IV for nourishment, etc. She had another few seizures, then stopped. She started improving. Met with the doc on Sat. morning (the next day) and discussed one of the possible causes being an inner ear infection (white count, vertigo, etc) and agreed to let them sedate her to check (necessary for the shape of their ears). They also found her to have a low thyroid count. Finally came to the conclusion that either a) she suffered an unknown trauma that resulted in swelling and damage to the thyroid-or a tumor damaging it, or b) she had a low functioning thyroid that brought on seizures and an unseen seizure caused her to hit her head on something in the house when we weren't around. It was chicken or the egg... ya know?
If it is/was a tumor, it would get progressively worse, and my understanding was that soon after going off the steroids it would worsen and we'd know. Well, we took her off the steroids and she improved steadily for several weeks.
Now, we go in regularly (every few months) for bloodwork (thyroid check and anti-seizure med blood levels)... she takes 2 meds twice a day (averages around $30/month for the meds?) and she is the same dog we had for the first 6 years of her life. :)) (HUGE grin)....
We were a terribly sad and suffering family for a week while we were in the position you are now... wondering and waiting and debating and realizing what Might be imminent. How to deal with it. How to talk to the kids about it. etc etc etc. My husband was a mess. Well, we all were. The kids were probably better than us adults.
But, in our case, we had a great outcome. If she had not started coming around on her own (after spending $200 at our original vet) we probably wouldn't have followed up with a 2nd opinion. But the partial recovery and then regression made us feel like we HAD to. We could not have afforded to do an MRI to find out a)nothing or b) that there was something we couldn't do anything about anyway. So I really understand.
When our (now current) vet, said "we can run a battery of tests" (not including MRI) to rule out some things and look for some other possible things that might be responsible... and do so for $500 including the medications/food/board/etc while she is here.... we just couldn't not give it a try. She was solidly a member of our family and "should" have had many years left to live with us. She was only 6 at the time! GSD's can live to 12 when taken proper care of them. She has no hip issues, etc.
My daughter (now 10) doesn't remember a time when we did not have our dog with us. She wasn't quite 3 when we got her.
Maybe talk to another vet to see if there are other things they can do to rule out other possibilities besides a tumor. That is what I was told was most likely. But it has been almost 2 years, and apparently it wasn't a tumor. With thyroid meds she is FINE now.
Did your vet check your pups thyroid?
Send me a PM if you need more moral support. We had to set a dollar amount too... and you just never know. There isn't a wrong amount.
Just talk with your kids too. Give them plenty of time to wrap their heads and hearts around this. Whatever happens.