PSA: The replaced child in 1st slot still gets full expTIPS
This is probably alrady known but I'll mention it just in case. The replaced child still gets full EXP after the battle. In stages n7,n8, h7 and h8 the first slot in the party is replaced with the partner child.
Although the replaced child doesn't participate in battle nor shows up in the final completion screen, they still get full EXP. Here I use lv 1 Bremen for first slot. The top 2 images are before doing h8 and bottom 2 are after.
As seen Bremen received 200 EXP. So you can just put any child on slot 1 to get free EXP while doing this ND. No matter how weak they are, they won't lower your chances of achieving 3-star clear.