I have this all the time.
Have always said they can have fruit and yoghurts at any time, as long as they eat their breakfast, lunch, tea, and supper. And ds1 just helps himself
So apples, bananas, tangerines, grapes and seasonal fruit is always available for them to graze. At the moment it's plums , apricots, cherries and strawberries. I spend a ridiculous amount of money on fruit, but it all gets eaten. Raisens too.
Yogurts-I buy petit filious normal ones and the ones with fruit purée in the bottom, mini fruit corners, occasionally the mini chic corners.
I don't buy crisps, cheddars, biscuits etc to snack on as they wouldn't last two mins I just get them for picnics and things like that.
Ds1 is 5, always in the go from the second he's up. Is tall for age and not an oz of fat on him. Ds2 is only 14 mo and if he sees ds1 eating he wants some too.
But if they don't eat their meals then they aren't allowed snacks, but to be fair it's not often at all this happens. I don't see the harm in yogurts and fruit. If they are genuinely hungry, as growing children are, they'll eat fruit and yogurt.