I understand wanting warmth underfoot in the bedrooms but large rugs on timber floors work just as well. However, if you're definite on carpet then stay clear of too dark or too light both of which are hard to keep clean. Too dark shows every speck of fluff and too light shows dirt and every stain. Mid colour range is best. Honestly while the kids are small I would choose a low priced nylon or synthetic in a square grid pattern. You can get these for about $29.95 a square metre made and laid. Mid grey, coffee or taupe goes with everything I think. I would put off laying anything high quality until the kids are older. My grandkids got Texta on my bedroom carpet and it's been a devil to get out. I'd use something hard wearing that you can replace in 10 years when they're more civilised.

Consider large rugs which can be updated as children grow and trends change

A Wood floor with a large patterned rug is what I would go for. Pattern is more for giving
You can get great rugs for kids and then later trade for something more grown up as they get older. Sounds like fun!

I raised 3 joeys and 2 cats on a Cavalier Bremworth pure wool carpet and never regretted it for one minute except for one colour choice. The living areas were in a deep brown which I would never recommend - as portpiro said, dark colours show every speck of fluff, dust, pet hair and anything else that comes into the house. The bedroom areas were an Arctic White and were so easy to keep clean - regular vacuuming and a deep clean annually or occasionally spot cleaning of any accidents. I had 40 years of wear from these carpets - probably won't get anything like that these days though.

http://www.cavbrem.com.au/tools/products.aspx

I think if you choose mid brown/grey colours in a tweed or a self pattern, you will find this does hide any stubborn marks, but with wool, I just didn't have any. I note now this firm has brought out a range of very nice looking carpet tiles. If these are easy to interchange, they could be very useful in bedrooms especially where only a small area of flooring gets wear while under beds and furniture stays pristine.

I also found when replacing my carpet recently that nylons and synthetics were not necessarily that much cheaper than wool but although they are more stain proof, they do build up static electricity that will attract dirt and make the carpet much harder to clean.

This is one carpet from their Turkestan range that I really liked for durability

What kind of carpets did you decide to go with, fionaslade?