Not sure on what your budget is , but as others have said I would look into a paid upgrade for your son or paying for a different lounge like Plaza Premium. I have yet to have my first CX lounge experience , that will change in 2 weeks, but I assume they will not let him in.

I understand you want to be kept together , but if your folks need that much attention when in the lounge then I wouldn't of recommended your son to be in a different cabin when flying. I have noticed J crew do not take a liking to visiting family members from another cabin ( just a heads up) .