Can I get my second child assessed through the CSA?

I have 2 young boys. One boy is 12 and one boy is 8. I have split from both mothers and have been paying both women money since we split up using a csa agreement for my oldest son and paying private agreement with my youngest son. Recently my oldest son has stopped visiting (stayed every weekend) because of his mother’s interference. Once he stopped visiting I stopped my payments as I wanted to force her to give me access to my son again. In response to this she contacted the csa who have made an assessment of my earning and are trying to get me to pay £96 a week out my net £380 under the old pre 2003 csa calculator. With my outgoings, which include my private agreement for my other child and bringing up another small child I really can’t see where I am going to get this money from.

Does anyone know if it would be possible for me to get my second child assessed through the csa and get moved over to the post 2003 csa calculator. I have read through the leaflet information and they have said that this would be the only way to move my first son onto the post 2003 calculator. Thank you in advance for any help you can give me.