Hi Goldendays
What a lovely idea. you can find some inspiration on the "Goypedia" section of this site. Scroll to the bottom of any page and click on the letter i - then click on Ideas for children. You can view the ideas (photos and comments etc) 16 at a time and there are a few members who've built tree houses and forts for their kids.
Hope it goes well for you, bet they'll love it.