I can only talk about AF PE as I haven't tried Delta's yet.
Kids won't be able to lay across the lap because of the fixed armrests. But they will have more space to lay down on their own seat, especially with the leg rest.
I know some don't share my point of view, but I feel much better after a long hawl in PE (even the hard shell seats) than in economy.
When booking, if you have the choice between A350 and B77- go for the A350, the recliner seats are much better.

For an additional 10K miles, it is definitely worth it.
Last minute upgrade from Economy to PE is often sold at 30K miles.