Same boat with our tribe - my 2 cents is a kids full body harness - kids under 40 lbs. have very small hips and if they should go inverted for some reason, story is they can slip out. Actually, even if over 40 lbs, I'd still probably want them in a full body harness till they're 55 - 60 lbs or so.
https: //www.mec.ca/en/product/5032-667/Bambi-II-Harness?colour=NOC02&gclid=EAIaIQobChMIy_qW0Pry5QIVEiCtBh30uAsEEAQYBCABEgJHPfD_BwE
Lots of different brands of these in climbing stores.
Have fun