Hi, just wanted to add in my 2cents worth.
My daughters are 18 & 11 (16 & 9 at BD). From the beginning we treated them differently just due to their age.
H and I agreed that Dnow18 could do her own thing and choose when she would visit. she has been spectactularly uninterested in visiting. She goes occasionally, only when H asks, never on her volition, rarely stays overnight. She has no interest in making her room there "hers". She pops around for coffee mostly. She has made it clear to him that she does not want to meet with OW while there. so far that seems to be working.
D11 is a more complicated situation, she finds it all very confusing. H wanted a more regular schedule with her and I agreed that was probably best, she needed the security of routine. So he picks her up from me Thursday evenings and drops her home Sunday evening. Fri evening to Monday morning with another evening mid week is, according to one lawyer I spoke to, a "normal" schedule here in Australia. He didn't show any interest in a mid week evening and I tweaked things to be Thurs to Sun - that way he has to do the school morning thing rather than just fun weekend stuff but D11 gets to always start the school week off from home here with me. He rarely makes any contact at all with her outside of this regular schedule. No phone calls, no turning up to school or sports events unless they fall on "his" weekend. I find this neglect of his little girl appalling but can't do anything about that. In the end, he is creating (or not creating) their relationship on his terms.
H doesnt like to have the girls together and I think there are a few reasons:
for a start its more work!
they do argue a bit and he has always found that difficult to deal with and left it to me. (conflict avoidance!)
Given the age difference they need different parenting...he can be D18's teenage buddy but has to actually be a parent to D11
D11 doesn't really want to go but has an OK time when she is there. I think she wants to spend time with her dad but doesn't want to be away from me, her source of security and unconditional love. We text a lot when she is away from me. I have already started talking to her about taking a little control of her visitation schedule, this will be difficult for her as she is very afraid that if she says or does anything that might make H unhappy that he "might not love her anymore". I would like there to be a gradual transition from the set schedule to a schedule that is flexible according to her needs and wishes and then the 'come as you choose" style arrangement that her older sister had from the age of 16.
That doesn't help you Magnite tho coz your kiddos are that much younger. I think a regular schedule is preferable. We all know kids do best with routine, after the upheavals they have already experienced they need to know what to expect. Also given your H lack of commitment to parenting so far I think you would start short and build up to longer once everyone was comfortable eg Sat afternoons at first gradually turning into a Sat night overnight. And unless baby needs mum (eg if sick) the kids are a package. All or none.