This is just one of those things that will inevitably happen when a child is on a strict diet. Even children with severe allergies will get caught out now and again. My DP and my oldest friend have peanut allergies and both have slipped up, even though they know what they are looking out for.
Gelatine is an easy mistake to make.
I am vegetarian because I never liked eating meat - the taste or the idea of it - and it has been about 25 years since I ate any deliberately. I have been given it by mistake a couple of times and have been very sick. Both times it was when eating out and I have been spitting teeth because it was an entirely avoidable mistake.
I have also been given things containing meat stock/fat etc by mistake and while I don't like the taste, it doesn't make me sick so I shrug and get over it.
It's a bit different for me because I'm not fussed about meat by-products, only meat itself, so there is no ethical issue, but I think you have to accept that mistakes will happen and teach your child to be alert.
[waits for someone to come and say "You can't call yourself a vegetarian if you eat by-products. Gets in first with announcement of intention to continue to say "I'm a vegetarian" as opposed to "I don't eat meat, fish, food cooked in meat oils/stocks, but I'm fine with rennet and gelatine]
I have a muslim colleague whose daughter gets given non-acceptable food now and again and they just say "oh well" and move on. They recently found out that coke or pepsi (can't remember which) contains alcohol traces and it was put on the list of "banned" foods - all that happened is that everyone stopped drinking it. There was no anger or hysteria.