How about testing the waters, next time they send over the children, remark to them that you are looking forward to when yours are older and you can send them to their relatives to stay with elder cousins and family.
What would they say?
The children are lucky that you facilitate these close family relations and they might not be aware of the customs regarding contributions. Is there another way the closeness could be encouraged, like video calls?
It is commendable that you want to take care of them all, and it is not the children's fault, and it is nice that everyone says yours is the best place; it is probably flattering, too.
But in making the decisions, maybe consider looking back 15 years from now, if the other family members never reprocicated and you will have a lot less savings for your own family and children in the end, or having to work more hours, would you now do the same?
At first your posts seemed to be only about receiving a contribution for food, but the more you explain, the more it sounds like everyone just sends over their children to yours so they have more free time. It's a complex picture between not being able to openly talk to them, your home being the favourite one and everyone sending you their children.
As an alternative regarding other children to play with, if your kids invited local children, these would go home at the end of the day so it would be a different situation and probably less costly.
I don't know how the situation would be in the long term, but the older nephews and nieces will probably at some point not want to spend so much time with younger kids, as they grow up, become teenagers and young adults; it could be helpful have more local friends of a similar age.