It sounds like they eat well during the day, so like a couple of other pp's have said, maybe offer them a bigger meal at lunch and just something small at dinner. Also, just a thought, maybe they just don't need to eat so much at dinner - they might not be hungry.
If you're really concerned, maybe having an appointment with a dietitian who's a children's specialist may help. They might tell you that everything is fine, or they might give you some ideas. Dietitians look at the course of a week when deciding if the child is getting enough of the nutrients/food groups etc and not just one day or one meal time.

Good luck