You might find it doesn't go down much, if at all.
Yes, he will now have to pay 20% of his net income in child support, split equally between the two children ... However before, he was probably paying (if through the CSA) 15% of 85% of his net income.
He would have had 15% taken off of his net income for the child living with him. Then pay 15% to you.
So for arguments sake, he has £300 a week net income.
Currently, they deduct 15% for his resident child, so he has £255 net for assessment purposes. He then pays 15% to you = £38.25 a week.
Now he will pay 20% of his net income in total = £60 a week.
Split between the two of you, £30 each.
So not too drastic of a drop.