Put the family on by day and count on being up all night. (Only joking but not too far from the truth)
You do not say how competent the crew are so it is hard to be definitive.
Best to have three hour watches as four without relief tends to make people very tired, and two hours doesn't give time for rest, but you are going to have to be up for crossing shipping lanes landfalls etc so try to plan that in. You are also likely to be called for reefing etc so do it early at the watch changes rather than leaving it until you have to. Better to reef before it gets dark, and accept the loss of speed, than have to try to reef in the dark when every thing will be unfamiliar. It is best not to split the change over as this means no one gets any sleep. If there are five of you, you could rotate the people so that you have two on but someone is always getting a six off spell, as long as this does not put two in-experienced people on together. If the people are really competent, and the weather is good, you might drop to one person on watch and reduce the spell to two hours.
When running the engine for battery charge, use it to make ground as well. The engine does not like to be lightly loaded and if you are running it might as well use it.
You will get a good response from the knowledgeable people on this forum.
If of the four to five people you have three people who are competent to lead a watch then split the watch into three - three hours on and six off is a brilliant watch system. Most people can end up more rested than if they were on land!
With two competent watch keepers you will only have two watches, and the easiest thing is then to have four hours on and eight off. The problem with that system is that four hours on is I think too long especially when it is cold or for young children.
Everyone down to the boat, pack and set off. Hurrah. The whole family helps out of the solent. Can't go to sleep there. "I'll be fine" says Dad as it gets dark, and all the others go to sleep. Dad helms through the night. the youngest child stays up later too, and they have a long long chat, largely with Dad asking if its okay bout them being the youngest, as if he could do anything about it, and smallest child not really wanting to be called "the smallest child".
Dawn of day two, and dad is dead tired. He goes below, wakes everyone up, and tries to sleep himself. The boat seems to whirl around and the curtians are no good at blocking out the light. So after an hour or so he comes back up, in a bad mood, and snaps at one of the kids helming. They go back down below for a nice snooze. That'll learn him. Mum makes a late breakfast for Dad. Dad eventually collapses in late afternoon for two hours solid kip. Kids help Mum all smoothly with no probs at all. Mum makes tea, rather wonderfully, doesn't wake dad, who eventually gets woken for late food at 9pm and starts checking things, perhaps tightening things by three millimteres which annoys the kids but they don't say anything. Dad again helms thru the night, and they arrive at destination in the middle of the following morning. Everyone has breakfast, and Dad has another snooze till ten minutes after all the restaurants are shut.
If you have 3 of the five capable of taking a watch you could use a rolling 2 on 3 off system. That is no1 and 2 do 1st hour, 2 and 3 do 2nd hour 3 and 4 do 3rd etc.
everyone has a go on watch and experience is spread about.
If you have to go to 2 watches make sure everyone still has a go on watch, including children. If not you find they play the noisiest games and music when you need sleep. Does not lead to happy ship. If you are interested and like diving there is a perfectly good Guns n' Roses CD about 12 miles East of Douglas following one such incident.
When cruising with three adults on board we used to have two hour watches which gave four hours off,but only overnight.The relief had the responsibility of making a cuppa for himself and the crew member who was coming off watch if he wanted one. This had the advantage of the new watch being fully awake and tuned in before taking over.
You could implement this basic system and have the less experienced members 'coupled' with more experienced members but in a somewhat more flexible way.
As we had a harmonious crew on board we had no need for formal watches during the day as it was cruising after all - not last trip of the Bounty.