Dinner at 4.00pm, and getting ready for bed at 5.00pm ... really?! Most kids are barely home from school by 4.00pm, and enjoying a snack, before having some activity, playing with friends/in the garden/watching TV. They have dinner later, and maybe get ready for bed around 7.00-8.00 ... on a school night!
My 3yo GC has dinner around 6.00pm, bath at 7.00, and goes to bed around 7.30pm. I'm not surprised that your GC are so lively and don't want to go to bed so early. Do either of their parents work? Surely, if they do, they must hardly ever see their kids awake!
If you're taking care of your GC whilst mummy & daddy go away for a couple of days, make it easy on yourself ... make sure they have fun/activities/stories/cuddle up to watch a movie together with them, and let them go to bed later, when they're actually tired. Your house, your rules. Why put yourself through the stress? If mummy & daddy aren't happy with that, then that's up to them. It didn't do my kids any harm when they went to stay with their GPs, which they loved to do! I was happy to let GPs do their own thing, as they were doing me a favour which I greatly appreciated! The kids were usually so tired when they came home that it wasn't a problem getting them to go to bed at their usual bedtime, it never disrupted them. Although, I can imagine, if they were expected to get ready for bed at 5.00pm, it probably would, and I can't say I'm surprised!