I'll diverge a bit from the opinions above, I would criss-cross the adults and kids. I'd put 3yo in (say) 2A, 7yo in 3D, and the adults in 2D and 3A. The pods are designed to be private so the only sightline for a buckled child to see an adult is across the aisle. Being able to see a parent would provide the greatest level of assurance for children. I was able to keep my 3yo happy during a 90 minute ground delay at Newark by reading, feeding him gummy bears, passing him cars, etc, from across the aisle. I don't think I would have been able to do that if I was directly behind him.

I get the argument regarding o2 masks, but really, the odds of needing them are so low, and there would be adults in front and beside the 7yo, so it's not something I'm concerned about.

If you think your 7yo is likely to be fine without an adult in sight, then I'd book an L shape, 3 windows (kid-adult-kid) and then the other adult across from the 3yo in the D/G seat. This is the way I've got it arranged for my kids in our first post-covid Signature trip, since I think my 6yo is going to be fine without an adult in sight and this way both kids can enjoy a window.

If you have a choice of aircraft, the aisles in the 330 are narrowest making it easiest to reach across them. Or for the centre pairs, the dividers go down, so that might be another good option since the 3yo can see a parent pretty easily. Again, if I were booking this option, I'd criss-cross, eg adults in 1D/2G, 3yo in 1G, 7yo in 2D.

If you're in a Dreamliner, 1A/K are well forward of 1D/G, so if you want to be across the aisle, book 2A/1D, or 3A/2D, etc.

Presuming it's not full, you can also ask to be seated in Premium Economy during the takeoff.