Help, we've been invited to a birthday party!

My twins have been invited to their first birthday party. It's for a little boy they go to daycare with. And since they spend at least 30 hours a week with him, I guess we could call him a friend. "Now what's so bad about a birthday party," you might ask. Well firstly, what should my kids wear? Do I need to put them in dresses or other "party clothes?" I kind of hope not because the party is at a park with a really nice playground, and dresses are, IMO not the thing for playing in the park. I hope I'm right cause I've opted for play clothes. I guess, if I'm wrong.... I did at least bring them clean shirts to put on after daycare. Secondly, what should we take for a present? Should I have the girls take 1 joint present? Or 2 separate ones? I just don't know. And the mom's at Twinstuff weren't much help; most of them said "sometimes we do one and sometimes the other." Gee, thanks. And how much I should spend. One twin mom said that she takes one gift for her 2 girls but spends more money - sometimes as much as $40. Holy Cow. I'm sorry but we just can't go there. And what if my selection is just plain bad? I could be labled the "bad present giver" by all the other parents at daycare. And no one wants that. So you can see there a lot of pressure when it comes to going to a birthday party. I just hope the girls have a good time.