How to get the all child records at a time per module using developer api?

How to get the all child records at a time per module using developer api?

I am having 16000 leads and for each lead i mapped 3 childs.
To get the child records we need to pass parent id(Lead id).
per 1 lead i have 3 childs so, I need to made 3 api requests.
per 16000 leads * 3 = 48000 api requests. So it leads my regular api request limit.

Is there any possibility to reduce the api requests to get the child records? If possible please provide me a solution.