5 pm is quite early to eat and then eat nothing till bedtime IMO
I think if they are skinny and active etc (high metabolisms) then what you are asking them to do is to eat MORE than they need at tea time - fill themselves up and make their stomachs feel over full - in order to make it through to bedtime
adults are quite good at doing this "oh I will have a big meal now as we won't be eating till late tonight" or whatever, but children aren't, and it isn't a healthy attitude to encourage IMO
I would give as much food as they want at dinner time and then have agreed things they can snack on before bed - bananas, kumquats, nuts, cheese, yogurts etc
I don't see why you need to be making them more food but I don't see why they can't eat something else 2 hours after a meal when they need a snack
tell them what they're allowed and let them help themselves