Hi MemphisDad,
The only thing I know that stimulates appetite is something not prescribed in the UK but I think they do in the US. Called Periactin...used frequently I think in children with Russell Silver Syndrome.
No, nothing wrong with being petite if its in your genes. If both parents are small/average then you''re not going to get a tall/beefy child out of them.
As I said before if height and weight are both in the same place on the centiles then it is likely that that is just where the child is meant to be. No point trying to stuff them with calories as they will just get fat.
If the child IS a poor eater though and underweight for their height then the dietitians recommend putting lots of cream into your cooking, high cal puddings etc.