The A350 (and the COI 77Ws) J seats have the middles that are close together. I expect on the few remaining NEV4 planes, that there are no restrictions on kids' seating.
I suspect that the rationale is a combination of having an adult close enough for safety (especially depressurization) and also to limit the noise etc. impact to adults not traveling with kids. The crews don't mind if adults and kids swap seats.