K.R.
Having a dog is great for kids- teaches them a lot of life lessons.
BUT- there dogs are NOT "affordable". They are expensive! If you get a puppy, you will have several vet bills over the next year to make sure your puppy has all the necessary shots. After that, your dog will have yearly shots. Each visit can easily be a couple hundred dollars. Your will also need to get your pet neutered (not only to prevent more unwanted puppies, but for males it will help curb unwanted behavior) Add to that-dog food, monthly heart worm pills, grooming, supplies, training, etc etc etc.
And on top of the expense, getting a puppy is like having a baby in the home. They need constant attention for the first few months. They will chew things, destroy stuff, make mistakes, cry in the night. I've also told friends who are thinking about starting a family to get a dog first. It's a good training mechanism. (Not to mention the automatic floor sweeper you'll have in place when babies start self-feeding!)
Dogs are big investments-please make sure you are ready for that. Especially as a single parent-your kids may say "we promise, we'll feed it walk it you won't have to do a thing" but it WILL be all you.
I know- I've got 1 dog, 2 cats, and a lizard that I somehow got conned into taking care of. I'm a big softie.