Your mother is apologizing for losing her cool, and then adds "but" - so she's justifying her overall actions.
I note she is NOT apologizing for going behind your back and doing something you specifically did not want done. THAT'S what the apology should be for, with a promise not to undermine you again. She has no reason to be pissed - she gave an internet-enabled piece of hardware to 3 children 9 and under. So now she's pissed and somehow it's YOUR problem that they went and used it? No children should be making purchases like that. You know it but she doesn't.
Her best bet - or yours - is to sell the iPad and put the money toward her bill. I realize she got her money back, but still. Then find a better hiding place for the iPad (giving it back to her means she'll let the kids use it when you aren't looking). (By the way, a girl in my family hid her diary in the last place her sister would look - under her sister's mattress!)
If your mother has done this sort of thing before, she's an ongoing problem. She wants to be the POPULAR grandma but undermining the mean, strict parents? Oh no, that's not gonna fly.
I would not listen to her excuses because those aren't really an apology. An apology involves a recognition of wrongdoing and some sort of assurance that it will never happen again. So far, I don't read anything in your post that implies that your mother has any clue that what she did was wrong. It sounds like she's mad at YOU for your "ill-mannered kids" - which is blaming it on you and absolving herself of any real responsibility!
So in that sense, I wouldn't accept the apology because it's not for what you're upset about. You can try to explain it to your mother, or you can limit her interaction with your kids.
Yes, they should have consequences for disobeying you, but they have the added confusion of another adult they love giving them complete permission to go behind your back. I can see that it was confusing to them. I'm not excusing them completely, but I think your bigger problem is your mother.
Good luck with this.