I want to get my children a playhouse for the garden. We have a 6x6 foot area for it but I can't decide on a plastic one (easy to clean and assemble) or a wooden one (looks better but might rot, need upkeep), or should I just buy a shed and the kids can use it as a den (and I can stand up in it and use it if they get bored with it).
Shed. I bought mine a small summerhouse type shed, with two doors that open out, with panes of glass that I replaced with plastic. The nice thing about this is that as you say, I can go in it with them to play (or more likely tidy up). And it will grow with them. We've put up lots of nice hooks to hang toys/equipment on, and they've got a little rug and furniture. We bought this one http://www.diy.com/nav/garden/garden-buildings/cabins-summerhouses/summerhouses/-specificproducttype-width_2_01_to_4m_wide/Lumley-Summerhouse-7-x-5ft-11987589?skuId=12498295 but I DO NOT recommend it as it is so badly made and flimsy. We had to re-inforce and add loads to it to make it reasonably sturdy, and you have to spend ages treating each individual piece with preservative before you use it. Right shape and size for us though! And looks den like enough.
A wooden one shouldn't rot if well treated and doesn't require as much maintenance as you'd think, provided you get a good quality one - you get what you pay for. I'm going to grow an enormous clematis up ours and plant some swishy grasses around it to give it a secret feel.
How do you feel about bugs? I really don't like spiders or insects so decided against a wooden playhouse as, from observing friends' ones, they often seem to end up covered in insects and spiders. We went for plastic as it's easy to hose it down when it's dirty and/or full of spiders. If you're not bothered by bugs then that won't be a consideration. Also, you might be able to get a very cheap plastic one to see how your children take to a playhouse (some children really aren't interested) and if they like it could upgrade to something nicer at a later date.
I love Little Tikes' play houses. If you don't mind having primary coloured plastic in your garden, they are indestructible and can be sold on after your DC are done with it. We also use ours to store small garden toys like footballs and watering cans.