A snack when they get home from school or in the middle of the afternoon is an essential part of their diet, even for children who are a bit chubby - if they go for too long between lunch and dinner they'll only end up eating more at their evening meal.
It's far better to have a light snack at 4pm and a normal dinner than a huge meal later on that they'll struggle to digest. Heavy evening meals can lead to weight gain, though it's only natural to over-eat if you're starving because you've not eaten since lunchtime.
Get into the habit of giving your child a snack in the afternoon, even if it's just a piece of fruit or a yoghurt.