Congratulations on your business endeavour. Do your research and your home work? Get the best help you can with the marketing and planning for your business. Finding fabrics and sewing the clothes (quality counts). Where will your costomer base come from? How will mamma's find you on the net? Make certain all of your legal needs for your business structure are met. Is the name of your business viable or does someone else already have that name?
There are many steps to starting and running a successful business. I would even think of offering the clothes at specialty shops as well as finding other children who fit the profile of your clothing line and offering it to them. Schools, daycares, churches, dance schools, etc. all have children there but don't really allow soliciting. Again how are you going to reach your target market?
Keep track of clothing trends and staples. Style counts too.
I think it is a wonderful idea but you need to be ready to shell out money from your own pocket and build up your brand while developing your business plan which will help you layout what direction you are going in, what you stand for, how you can expand your business, what resources you need, what capital you will need, how many people you will need, room for growth of the business and how to dissolve the business.
Success to you both for this passionate idea.