This might be a first world problem to us adults but to an 8yo that went to school with $5 to buy her Mum a Mothers Day Present and was one way or another convinced to give her money to a charity is a big thing to an 8yo. My DS is 7 and a half and I bet my $5 he would have given the money because he knew it was the right thing to do but would be absolutely gutted that he didn't have anything left for me (or his Dad on fathers day).
Kids understand parts of things, they don't always understand the consequences, they are kids. Yes girls in Nigeria are kidnapped but that's not something I'm teaching my 8yo just yet - they are worried about first world problems like this because they are kids.
I'm sorry OP I think it was really wrong of the TA to do that, she may have had the best intentions but do it with the older kids who fully understand the ramifications of what they are doing with their money, not the younger kids.