Welcome to the DIS!
We have arrived around 45-30 minutes early with the F! dining package and sat in the w/c seating at the back. I can't say whether that's typical or not since we only did it once, and I have not done it with FP+. You may want to arrive a little earlier with a party of 5 (we only had 3).
Note that you likely will only be allowed to have 1 person sit next to the wheelchair, and the rest on the row in front. The cut-outs are set with just 1 companion seat per w/c space. I suggest you have one of the kids sit next to DH, while you sit directly in front of them with the other 2 kids. Once you arrive and claim your seats, it is possible for you to walk around with the little guy pre-show while the rest of your party saves your seats. It will be crowded, though, so you'll need to keep hold of him.
Enjoy your vacation!