Is it permissible to dispose of in-kind aid entrusted to you by dividing it among the beneficiaries in installments, or by transferring part of it to another eligible recipient, knowing that you are able to compensate for the shortage?

Trustworthiness is a great virtue in Islamic law, and it must be fulfilled in the manner requested by the one who entrusted it.

What you did by giving some milk to the other woman is considered a betrayal of trust. You are obligated to guarantee it and hand over the remainder to the child's mother so that your responsibility is discharged.