Moore River / Guilderton is great, it's only an hour north and you can hire a holiday house for stay in the caravan park. It's very popular at Easter & Xmas though. We go yearly with a bunch of friends. There is the river & beach, a golf course and small shop, you need to take your food with you etc. But its such a lovely spot.
We lived there for 6mths and just managed one holiday to Busselton and Margaret River and thought it was a fabulous family holiday spot. Would go there again in a heartbeat.
I tried to arrange another holiday north but I was so shocked by the prices to get the family to Coral Bay or Exmouth and the accommodation that I have to say I gave up. Maybe I am so used to my cheap old SA that I just couldnt cope with WA prices. I did price up a Bali holiday and that made a lot more sense.
Busselton, Dunsborough, Yallingup, Margaret river, Denmark, Pemberton, Bridgetwon (but this time of the year would want to be near the beach). Kalbarri. If you feel like venturing interstate, Tasmania was great. so much to see.
What sort of holiday do you want? I'm assuming that since you don't want to fly interstate or overseas that rules out Broome and the Kimberley. Coral Bay and Exmouth are awesome - especially with kids - but a few days of driving. Augusta and Guilderton have lovely safe river mouths for little kids to swim and play in. Both even better if you have access to kayaks or similar and both have good camping grounds. Windy harbour has spectacular scenery and is a really nice and safe little community. Pemberton is nice for inland stuff - lots of forests, hiking etc.
But my recent favourite is Ballingup - lots of lovely little chalets - beautiful almost alpine scenery - the town is interesting with some great antique / second hand shops (and the most spectacular jewellery shop in WA I think!), Golden Valley tree park.
I had one night in Rottnest this week and it was as good as a week. The air breathes differently there. Everyone smiles all the time. It's a bit expensive but if you can spring for it then well worth it.