1 - Would you buy a t-shirt for yourself?
No. Not a big t-shirt wearer and already have too many free ones from events and stuff like this that I don't wear often.
2 - If so how much would you pay for an adult t-shirt?
N/A
3 - Would you buy a t-shirt for your children?
Yes, definitely. I would also consider buying some for friends' kids who are trying cloth (babies yet to be born).
4 - What sizes would you buy?
1yr - 18 months for DD
NB sizes for new baby and friends' babies
5 - How much would you pay for a children's t-shirt?
$15 or less
I wouldn't mind either way if it had the TNN logo on or not - I mean, it would be cool to have the TNN baby on it somewhere, but if it didn't I wouldn't be put off buying it. I would think a t-shirt for kids would be much easier to do in terms of sizing and style of t-shirt to please most people. Maybe offer a different sort of promo item for adults that is smaller and cheaper - a cool button or bandana/scarf type thing, or hat? Then dads and mums can get them, you don't have to worry about sizes or styles and people can wear them how they want. Or even a baby sling! That's getting a bit out there expense-wise though, and not everyone would go for a sling.
Ooh! What about getting a funky adults underwear company like thunderpants to do a RNW/TNN branded pair of undies for adults?

Now I'm getting silly. But it would crack me up to see something like that on a stand and I'd buy them!
It would be good to see some profits going to TNN but I think it would depend on how much that made the t-shirts. If you can do a small mark up for that cause and they are still $15 or less, then great. If they are more than that, I'd stick to the wholesale price and concentrate on getting awareness out there this first year. Getting it to take off and become popular is the key to begin with, fundraising can come in later years.
I really like the caption competition idea, I think it would work best for the kids t-shirt rather than the adults one, if you do one.
Best of luck with the t-shirt organising!