As I have made multiple garments over the last 2 years I am pretty aware over my current overheads for materials and my time etc
So how does that translate into the cost per item when manufacturing on a small scale?
What would it need to sell for to cover other overheads, so materials, the market stall, wear and tear/maintenance on your sewing machine and scissors?
How long does it take to make a single item?
If you scale up and make 10, how much can you reduce the time taken for things like cutting out?
If it takes 2 hours to make a single item (no idea, just a guess) then that's the best part of £25 at NMW before you add on time for selling, sourcing materials, doing books/taxes/banking etc.
If you spend 6 hours at a market stall, plus setting up/travel etc, you'd probably need to take at least £200 a day, likely more to make it worth your while. Make sure you know the cost of taking payments (card machine, banking fees) and the substantial cut that any selling platform will take - they're not primarily there to provide you with an outlet for selling the clothes, they're there to take a lot of your money from you.
You've got a few potential customers, but they'll only buy so many and what happens when their DC grow out of the clothes and they donate them to local charity shops or pass on to friends and families, reducing demand for new items.
Plus marketing. A lot of success these days is due to your social media presence. Don't underestimate the time taken to maintain this and stand out.
Also don't underestimate the impact of a bad review, even if it was due to something outside your control like postage issues. Not meeting a certain level of 'perfect' reviews can mean that your items are not visible to potential customers.
I'm not saying that your idea won't work, it's probably area dependent, how many people you have locally who are willing and able to pay a premium price for DC clothing, that's likely to be significantly more expensive than standard retail and are still going to end up covered in bodily fluids and mud, food and whatever else they pick up when crawling around and playing, before they grow out of it.
You could view it as a hobby that covers some of it's costs and see where it goes. But obviously that depends on you being able to spend your time in this way on top of caring for DC, running your home etc.