S.W.
Yes, you can wear clothing from stores like aeropostale, but only if it's appropriate for someone of your age and mothering status. I'd skip the graphic tees and anything that advertises the store - those are just goofy on anyone over the age of 20. Yes, their shirts are cheap, but is that the look you're going for... cheap? And if they can afford to sell them for that low a price, do they really last that long? I stopped buying Old Navy clothes years ago because they would fall apart. For a little more money, you can get something that looks nicer and will last longer.
I love watching "What Not to Wear" on TLC and you are just the type of person I've seen on that show - a young 30-something mom that just lost weight and has a nice body to show off. If you've never watched that show, give it a try.