Will the parents agree how to split it?  If so, the father can claim 100% and the mother can claim 0% (but you still need to fill out the allocation sections).  If they can't agree, yes, in the Allocation section, you need to enter 0.67 on the father's return and 0.33 on the mother's return.

The 8962 will still show a Household Size of two for the father, so Premium Tax Credit factors that in.

For the future healthcare applications, the father is required to tell the Marketplace that he will not be claiming one of the kids (if that will continue to be the case).  The kid will then receive their own 1095-A.

You said "because he is worried Mother will drop the ball and forget to have insurance for the daughter".   Does the mother also have insurance on the kids?  In other words, are the kids double-covered (or eligible for insurance through an employer)?  If so, you need to change the SLCSP in column B of the 1095-A.