Hey don't beat yourself up, we are all human. I've had almost exactly the same situation this week (family Kindle not ipad) and I am not proud that I snatched it but I did 
snatching is a problem, no matter how fedupyou are. We wouldn't tolerate our children snatching things from each other, so we shouldn't do it to them. I apologised to ds1 after I had done this (he is six) and he accepted my apology but was clearly upset that I had lost my temper.
I think screens plus tired children is very often a problem. They are mentally shattered from the new term starting but the screen keeps them awake.
I use loss of screen time as a consequence as often as I can. My kids are so much happier, chilled out and nicer to be around when they have been playing outside in the old fashioned way, or messing around with their toys / chatting to me / helping me cook etc. If I put the effort into providing a non screen play opportunity, it is always worth it - they are happier and I feel like a much better mother!
I've been so busy with diy and painting recently that they have watched far too much tv. I know it's short sighted of me too let them do it, because we always pay for it later!
Anyway, we live and learn, and I don't think a few unfortunate incidents like this will damage them for life.
My plan is to organise a decent chance to play in a non screen time way every day after school - sometimes high effort like getting the paints out, and sometimes low effort like letting them dig holes in the garden or make a racket on the drums - is working for me every time I do it and their behaviour is so much more lovely. Hope this works for you too.