Children like routine in many areas of their life, including around food.
It’s much easier for them if they know when to expect a meal or snack, so having a routine that you stick to (it doesn’t have to be to the minute!) will help them to understand when food will be available as well as cope with waiting if they’ve eaten just a little or refused a meal. That’s not to say there will be no tantrums or fits of hanger but they will reduce!
Having a routine will also mean that they aren’t too full from the last snack to eat at mealtimes and should reduce the constant demands for snacks.
If implementing a routine is a big change for your child, you can talk to them about the schedule so they can understand it. For example, you could explain that they’ll have breakfast, then it’ll be time for trampolining after which there’ll be a snack. You’ll do the shopping and then have lunch. After you’ve picked up big brother from school, you’ll have a snack. There’ll be time to play and watch TV before we have dinner.