How to BLOCK GIFS from apple watch

My son who is 7 uses an apple watch as a communication tool as he has mutism. This is a key tool for him however we need to use iMessage. Everything is blocked accordingly and he only has access to what is needed. He uses this at school to function. As he is only 7 years old - the GIFS that he can access and search in the iMessages are extremely explicit and vulgar. He can't be able to search and see these images at all at this age and to have access to at school. Blocking imessage is not an option as he needs full acess to messages to get help from his parents at anytime. Please help - this is his lifeline and this is SO disturbing the images he saw. We need to remove the gif button entirely. I can't find anything to do this.