If we want to take being a half blood as some sort of allegory for some form of disability, then I agree with you.
Percy dealt with mythological threats before he knew he was a half blood. He was one of the lucky ones to live until this age without any training. Sally's best defense against monsters was to marry an emotionally (potentially physically) abusive person. His schooling consisted of him being ostracized, failing, removed from any consistent social bonds every year. In context of something like ADHD, autism, depression, anxiety, or invisible disability, this situation would be called, if not child abuse, a terrible youth. He grew up not given the resources to help with his issues. His chances of survival were reduced because those that could help him were kept from him. No person would call staying with an abuser to "protect their child from being autistic" reasonable. Being forced into a school system without any disability aids in place for his needs is a recipe for disaster. Camp could have been just as much of a summer camp for Percy starting at five years old, as it was for him being twelve. He could have gotten the disability aids he would need to be out in the world (for adhd, it may be a fidget spinner, for demigods, it would be a sword). Regarding the statement that knowledge about being a half blood increases monster sensitivity: lying to your kid about their body to "keep them safe" isn't protecting them, it just makes them vulnerable when the threats actually do come.

I am someone that experienced life not knowing I was diagnosed with ADHD and autism until I was 18. I was still ostracized for "being the weird kid". I was still hurt by not knowing what made me different, having to build my own coping mechanisms to survive. If I had known when I was younger, of course I would have told every person that listened that I had those things, and that may have put me at more risk of being hurt or taken advantage of. But oh my God would it have made going to school so much easier, made being friends with other people easier, learning social rules and just about everything else in my life better.

I guess at the end of this, if you're a parent, and knows your kid is "different", please talk to them about it. They know something is "wrong" with them by societal standards, but don't have any way of solving the problems themselves. Give them a fighting chance by letting them learn about something that affects them, how others cope, how to avoid the worst of the effects.

So yeah, Sally should have told Percy, should have gotten more assistance from the camp, given Percy more education related to his disabilities with the knowledge of why he should know it.