My son uses ex lax chocolate on daily basis due to health need.
Can we use on Pesach?
I haven’t seen it listed on the permitted list of medications, therefore it shouldn’t be used, even for a child, especially if he has reached the age of chinuch. There are a number of laxatives that are indeed kosher for Passover. You can check the lists of the CRC, OU or the Star-k, for laxitives that are kosher for Passover.
Chag Kasher v’sameach