My take:-
1. The child MUST stay with the mother at all costs at least until he/she is 18 to avoid the usual mistreatment stories wa mama wa kambo.
2. Find a method/neutral people who can put the two dudes and galz together to talk thus making sure all stakeholders are in the know, disagreement galore route nonetheless must be traveled.
3. Child support amount can be agreed this to be managed jointly by mother and baba wa kambo albeit na vita kila saa(mtoto asijue storo or ajue baba ni step dad)to avoid seemingly discriminating situation leading to isolation by the half-siblings.
4. Finally if party(s) are religious the holy bible or noble quran can give fair judgments tafuta pastor wa maana ama ustadh/sheikh for grounding of agreements.
Msisahau mtoto! the interest of the child/children in the mix come first don't spread the rot into the children of both families BELIEVE ME I KNOW!
I AM trust in GOD, I AM belief in THYSELF