Being summer time it is extremely hard to do a semi-formal for boys that isn't considered winter wear. Maybe a nice polo top with khaki shorts and a nice pair of shoes or sandals. I would focus on his outfit first because it will be easier to match her to him than the other way around. I like Belk for nice toddler/baby boy clothes they have carter and so many other nice brands. I found the cutest plaid brown/cream/blue shorts and light blue polo top that I paired with sandals for his 1 year old photos. It was too cut and they turned out so well. It may also be cute to have them both go barefoot in their pic's this way you wont have to worry about the added cost of shoes.
Also in your last question you asked me how I made my twins birthday invitation:
I have a really nice photo paint editing software and I custom make invitations, birth announcements, etc. If you go to your YA profile page and then click view pulse then scroll down for comments.I put my email address there if you would like me to custom make an invite for you.
All these shops have real cute stuff...almost certainly it comes from China, with some items from Thailand, Malaysia, Philippines, etc. Not many of the brand names were listed, but I wasn't able to find anything when I googled the ones that were. You could try contacting one of the sellers, I think that's a longshot because obviously they're in a competitive biz and wouldn't be likely to give that info out. I googled "china wholesale clothing" and it came up with lots of good matches. I'd start there. Good luck.