Entry Level WWI and WWII Models for 8 Year Old

So you have a couple of options here. Two manufacturers make some really nice entry level kits.  HobbyBoss has their "Easy Assembly" kits and Airfix has their "Quick Build" kits. Both are marketed towards the younger model are just starting out and are inexpensive. The hobby boss kits do require some painting but the airfix kits are pre-painted. The Airfix kits actually assemble like LEGO's. Do a little research on both to see which one would be a better fit for your right year old.  You can find them for sale here: https://store.spruebrothers.com/