You need to first establish that your base is not in a wet area and is basically level - you need to build up concrete underneath and then put slabs down (not forgetting your waterproof membrane under the concrete).
B&Q wood things are a complete horror to assemble and you will need six stout friends (and one bottle opener each) to get through the experience. However local handypersons are quite cheap compared with the years of counselling so you might want to go that way
We ordered one about 2 wks ago from b and q,phoned them today to check on delivery and it will probably be another 3 wks yet,just to bear in mind if your dd's birthday is imminent!
DH came home today with bags of gravel and some breeze blocks(if thats what they are).I'm hoping it wont be too diff to assemble as he is NOT a diy er...
You can bed your slabs on dry sand/cement mix as long as you level the ground well first, check levels on slabs when down.
Or you can use hefty wood bearers (at least 4"x4"), well (pre)treated with preservative, although I always like to give them an extra few coats - you can get your local joinery sales to cut them to size for you. Just clear the area, level it lay a membran and put the bearers so they run accross the supports under the base of the wendy house. The bearers will easily outlast you DCs interest in a small wendy house (my dd has had to duck to get in the door of ours since she was5!)