Travel around Cambodia

I think the only way from Sihanoukville to Btb is via Phnom Penh. Long day of travel. This will give you some idea (doesn't cover all bus companies though): http://www.canbypublications.com/cambodia/buses.htm
Actually, if you can find a private taxi that will do it that may be a better option and cost wouldn't be that much more than buses since there are 4 of you.

Re the boat trip fromBattambang to Siem Reap, I did it a few months ago. I bought the ticket directly from where the boat leaves near the Battambang hospital but you can also arrange through hotel/guesthouse.1 trip/day. Cost $20. Departs 8 am. They will tell you 5 or 6 hrs but it's more like 7. It's an interesting chance to see life along the river. It's better going Btmbg to SR rather than the other way round because you're going with the current.
When you get off at SR you're still several km out of town and you'll be pounced on by desperate tuktuk drivers eager for passengers. I paid $4 to get to town.

When you say you're heading south to a homestay, is that in the Takeo area? Certainly not crazy in the wet season and in any case Takeo hasn't seen that much rain. If it rains it's usually fairly short and there's always a spot where you can sit it out. Good way to meet the locals. The road is not good though. It's not wide and carries quite a bit of traffic. If you're not familiar with Cambodian traffic you may not find it pleasant and taking your kids along could be a challenge. Good luck.

I'm not usually one to express concern about posters trying to do "too much," but I would recommend that you consider removing something from this itinerary. You are looking at 9-10 hours of traveling to get from Sihanoukville to Battambang.

3 nights in Phnom Penh is fine, 4 nights in Siem Reap is fine. Otherwise, you are looking at 1-2 days of cycling to the homestead, then continuing on to Sihanoukville, staying only two nights, then 9 hours of travel to Battambang, staying just two nights (only one fully day?), then the boat trip to Siem Reap. When I did the boat trip (10 years ago) it took 10 hours, I think that was low water season though, I remember the boat bottoming out a few times and breaking down.

You might be looking at ten hours of traveling to get to Battambang, stay for a day, then 7-10 hours to get to Siem Reap. I think you should consider reducing the homestead nights and/or cutting out Sihanoukville or Battambang. Both towns are quite missable, you might want to pick or or the other depending on what appeals to you.

I agree with #2, it is a lot of travelling, personally I would cut out Sin'ville as I enjoy the boat trip between BB and SR (again a long travel day), either way I would cut out one or the other.

I'm just wondering about the cycling side of things. Assuming they have to hire bikes, what are they going to do with them after getting to the homestay. Especially as they said they were heading to Snooky. Ride all the way?

Actually, if you're staying at a homestay 80km south of Phnom Penh you're halfway to Kep/Kampot. You might want to consider skipping Sihanoukville and spending a couple of nights there instead.

Thanks everyone for your responses and tips. We are rethinking the cycling piece (kids on busy roads concern) so that may buy us some time. The reason for Sihanoukville is that we are visiting family friends there who have a guesthouse. Is it feasible to visit Kep from Sihanoukville. The friends have also suggested a trip to an island - any experiences/recommendations in that respect?

Thanks again,
Chris

Your time is limited but now you seem to want to add Kep and an island. It makes it hard to offer advice. Should be possible to get from Sihanoukville to Kep but your homestay is halfway to Kep already. Makes more sense to go to Kep from the homestay and then from there to Sihanoukville. Or are you planning to cut the homestay altogether?