That's why I try not to mess with kids bikes unless I get them for free or they are the oddball stuff like little Harley bikes or Choppers. Other than those I start with 20" BMX and up from there. Also I try to stay away from 24" as well, just takes forever to sell them. I normally try to deal primarily in 26 inch mountain bikes, cruisers, once in a while road bikes, but mostly I try to stick with what sells. Recumbents, tandems, cargo bikes, oddball stuff, antiques, and BMX.
My biggest problem is I fall in love with a bike too easily and either end up keeping it or spending too much to get it.