I think it's always a cool idea to do something different, but definitely look at the costs before you get sucked into it. I've been down this road already and knowing your market can be the easy part, but making the clothing and getting traction is so hard.
Also, for girls in the park we are a small breed :( so you may even entertain making something that is also suitable for boys. I'm not anti women's clothing believe me, I want any women's brand to succeed - but you really don't see too many girls rocking the already existing chick brands... I think some girls tend to like the boys stuff a lot more, bc it's trendy and "core" (ugh I hate that) even if it's like saga for example that has womens pieces.
This is just my opinion tho, look into creating an LLC depending what state you're in (mine was only $50 with paperwork & lots of research if you don't want to do it through an attorney or legalzoom (but I also spent time in law programs in school)) and look at Shopify if you plan to do an online store.
By no means do I want to discourage you or sway your dream!! You can do anything you want, just have awareness about all the moving parts :)