Marine Infantry security clearance. Please Answer?

I have been to MEPS. Passed everything. I leave for Parris Island this may. I neglected to tell my recruiter nor MEPS that I was voluntarily admitted to a childrens mental hospital 5 years ago. I signed an infantry contract. What level of clearance do they have? I heard they only check criminal and debt history. If I do not put that I was in the hospital on their questionnaire, Will they still see this?