Are these kits good as birthday party activities?

Yes, mega-kits with many premade pots (FunKidz 33-cup, the 126-piece Ultimate kits, Elmer's Celebration) come up repeatedly in r/Slime party-planning threads as the easiest option, because pre-measured pots avoid the chaos of measuring glue for a room full of kids. Plan on one pot per child plus mix-ins.