Courts will say you knew about first support order and then went on to have more children.
Why are you up for modification? Ex #1 needs to show why she needs modification. (change of circumstance) so what is her reason?
As for the tutoring, maybe you can get it through the school district, health insurance, peer/teen/college student. Here the school district pays for speech, OT, PT, and parent counseling from birth to 21. If child goes to school, the services should be during the school day and you can request summer services. Also, child should be able to get extra support at school for reading and math if those are issues for him. If the IEP is not showing growth , then call a meeting and add services (at their cost) or go through medical insurance and pay copay's instead of paying per session. He can be getting services every day at school (reading 5x per week, speech 3x week...).