So confused ! Can''t seem to win

I''ve a real dilemma, despite contacting csa couple of years ago when problems arose with ex not paying maintenance , for the last few months we have had an informal agreement following mediation. Therefore I didn''t give the csa another thought until now when he has stopped paying again and won''t reply to my emails. I''m scared of rocking the boat especially as my eldest son is now living with him therefore I''ve only just realised the csa have all children down as being with me when only 2are now ( he pays me direct not through the csa) neither myself or ex have told the csa as it was a temporary trial at first with my son being there. Now I don''t know what to do , as the change should be backdated but meanwhile we said in mediation we wanted not to involve the csa.
For the sake of receiving 200 a month I''m even contemplating leaving it as whèn he is paying anything my ex thinks it gives him power and a right to demand. I''m a) afraid of looking bad to the csa for not telling them sooner about the change despite not wanting to use them anyway b) afraid of rocking the boat with ex who then takes it out in me and the children by being difficult meanwhile Christmas is on the way and I''m not well paid as it is so I need every penny for my children

Is there a CSA assessment in place? If so if you haven''t told them to close the case then the last assessment stands and arrears will acrue.

Telling the CSA and child benefit that one of the children is no longer living with you is a legal obligation so you should do it ASAP.

You also need to bear in mind that you will be liable to pay maintenance to your ex for the eldest child who lives with him.

Thank you for the reply, yes I''ve told child benefit but as we agreed to arrange directly through mediation I genuinely never thought to ring the csa. Daft I know.
I just want to do the right thing and have given ex the maintenance figure minus the amount I should be paying him for eldest child.