My older daughter is going through a phase where sometimes she just isn't hungry at dinner. She is 10. This is the first real problem we have had with the kids eating. I can't let her have juice, milk or anything to drink for a while before dinner. I have found that she drinks it down and fills up on it. She will do this if she has a drink with dinner too.
I also don't let my kids have too many snacks during the day now. They get meals and yes they have snacks here but not many. Snacks may be cheese, oranges, cereal and things of decent nutritional value. Rarely do I let them have chips for a snack or candy.
Another thing to remember is kids eat more during growth spurts then they will when they are not in a growth spurt. There are times my 5 year old eats more then my 10 year old. I now know that means I will be buying new clothes :)
If you can limit fluid intake before dinner and also limit how much of a snack for at least an hour and a half before dinner, see it this helps. Another idea is having them help you make dinner. Many times kids get excited to eat what they helped you make.
I wish you the best!