If you are relying on school to teach the lesson of charitable giving them you need to have a word with yourself. School is not the only outlet for charitable giving. You can do so on your own terms quite easily.
It is for a lot of parents who don't know, don't care or don't have the time or money to model it. Children also learn better from personal experiences outside of the home a school can provide.
Indeed I think it's important to learn the lessons of saying no when you feel uncomfortable with something rather than going along with peer pressure and being willing to go against the grain for something you feel strongly about. Charity giving due to this need for conformity or not rocking the boat is not freely giving - it's giving under duress. There's much to unpack in terms of consent.
Then do it consistently or don't do it at all. But I suspect its only when the word 'trans' is in the mix that 'suddenly' it becomes about 'consent'. I'd bet my house that the OP wouldn't be the least concerned otherwise.
I am getting exceptionally frustrated by the sheer number of threads on MN where the OP has a perfectly legitimate and fair reason to say no, but they are frankly too wet to do it. Sometimes at significant personal cost.
It's deeply unhealthy.
This is reality of the human condition though. Most people are always weighing the trade offs of when to speak & when not to so its hardly exclusive to trans rights. We are all individuals after all & as such need to accept our differences if we want to live together. You just can't have it both ways where you can dismiss another's rights to values & expect them to be civil with you.
And its not as if there aren't other avenues to advocate for/against a cause that they need to disrupt the school system's basic responsibility of teaching children about the value of charity & social cohesion.
Ultimately this issue is about framing. It's not selling out your values to accept others have a right to them as well. Now that's a valuable lesson the OP could teach her child about this situation.