How to increase apetite in toddlers?

No need... You dont need to worry about this in most toddlers. There are very few reasons for children to avoid eating to the point of poor growth. Ask your doctor to see your child's growth curve. If they are gaining weight well, then why are you worried? Truly, for toddlers, if they eat five solid meals a week and then little bits here and there, you have done your job. If food is available, kids are fine.
Nothing. Toddlers are usually picky eaters. Some days they may eat well, and others not so well. I would make sure your child is not getting more than 8 ounces of juice in a day, and no more than 24 ounces of milk daily. I would also recommend a 3 meal a day 2 snack a day schedule. In addition, it may take offering a new food 15 times before they try it.