Don't worry, most children won't starve themselves! It could be that your child has irregular eating habits, ie one day he or she eats very little, the next day he or she eats everything in sight.
Don't force your child to eat, and if they have a poor appetite at meal-times don't try and make up for it by giving them junk as a snack. To make sure everything is normal, chart your child's growth and weight.