If you could only get two seats in a premium cabin, then you & SWMBO could go in F or J then the MIL & 2 kids could go in J or PE (gasp) as on the A380 you could always meet up in the bar subject to CSM's approval & J & PE is all on the upper deck.
I suspect it may be better to 'take one for the team' put SWMBO & the MIL in the higher cabin then you fly with the kids in J or PE otherwise you'll never be allowed to forget it.
Best not to get the MIL offside as we don't want her feeling like she's tagging along as the hired help or a nanny. :shock:
I'd be happy for MIL to go Y!