I think I've answered everything below, maybe not in order!
Why is he a SAHD?
He is a SAHD as for their family it's best financially but obviously not for his older kids!
Can't he offer to have them more to make it up to 50%?
He probably actually could go closer to 50% and would... my maths isn't fab but across the year with weekends and holidays etc but not on a week-to-week basis due to school and probably not in the best interests of the kids.
Who are you in this scenario, OP?
Mum (of the older kids with exH)
How old are the children?
Kids are pre-school and infant school age.
Could you talk to him and his wife c this?
They don't think they're being unreasonable.
Do you think they did this deliberately to avoid maintenance?
Actually I think him being the SAHP makes sense for their family but obviously that's very selfish for older kids.