My autistic 3-year-old is in a chiloni gan due to his special needs. They’re having a Tu B’Shvat party with parents and will be planting at the party. Is it a problem for me to attend if I don’t plant? Do I have to stop my son from planting? Do I have to speak out against it? (I don’t mind explaining why I’m not planting, but I don’t feel comfortable giving anyone else mussar.) Is it better for us to stay home? The gananot will feel really bad if we miss it, but if that’s the best solution, I’ll do that.

I strongly suggest telling the gananot asap (!) that Misrad Hachinuch and even the JNF don't do planting during the shmitta year, and that's the official policy. If it doesn't help, than you don't have to stop your autistic 3-year-old from doing so, nevertheless, it would definitely be preferable for you personally not to be around when they plant for that would show acquiescence with the forbidden action and may also be mar'it ayin (maris ayin) that others may deduct from your action, that it's ok.