The 8mm Habotai is the best weight for playsilks. 35x35 is a pretty standard size but you can also do 45x45 for bigger silks. The 10mm Habotai 45x72 sarongs are also great for big canopy size playsilks.
Quite honestly, you could do them in any size you want, they'll be just as fun
The 30" x 30" size is nice as well in 8 mm Habotai (I buy prehemmed scarves) particularly since you have a couple of younger children. I found with my toddler that sometimes the 35" x 35" was a bit on the longish side if being used for a cape or head-dress or anything (since she's only 3' tall herself!).
Thanks for the input. I got some 21x21 AND some 35x35. We have dyed some 21x21 to play with. Thise are great for waving trough the air while dancing. The 35x35 will be dyed as a craft at dd's borthday parts, and each guest will take theirs home as a favor