OP, you have my sympathies. So many women are stuck in this position of needing to be their children's main carer and unable to pay for childcare because they don't have paid work, or their job doesn't pay enoughto cover the costs of childcare.
Please believe me, knitting (lovely) hats and charging yourself an absolute pittance is not the way out of this bind. 
No, I don't think £15 is unreasonable for a hand knitted hat actually, not if you factor in your time. BUT as others have pointed out here, your perfectly nice hats aren't different enough to justify charging more than fecking Primark.
And incidentally, all you thoughtless people who are parroting '£15, bloody hell I would pay a fiver in Primark', Primark and other bargain basement retailers are helping us along the road to utter environmental hell with their disposable one-season fashion and clothes literally cheap enough to bin after a few wears - in the privileged Western world anyway). 
You can also bet that Primark and their suppliers don't pay the makers of their crappy cheap clothing anywhere approaching £3 an hour.
That rant about disposable fashion aside, OP, can I give you a few tips? I have a craft-hobby that I take to small local events and a local market. It really doesn't make me much, it does make me fairly happy and I have a high needs son with autism so 9-5 jobs fitting in around his erratic needs and challenges are tricky for me. 
A. I would say, find a decent outlet for this. Don't put it on facebook marketplace, there's loads of twee shite on there and I don't think people search it for handmade anyway. See if there's a local market that makers go to. There may well be one and stall fees are often low. See if there's a pop-up Etsy shop local to you (do a google) where you'd have to pay a commission/proportion of your asking price, but work out if it still works for you. Beware 'craft shops' where rents are often very high for the shop owners and they consequently have to charge a very high percentage of commission to their crafters - I once had jewellery in a lovely local shop that hosted a lot of fairly high-end craft and art - she charged us 45% commission which meant I had to jack up my prices so much that not a lot sold! If there are any local school fairs coming up, ask what their stall fees are (£15 used to be the going rate but it's shooting up unreasonably - you may be lucky). Remember, your outlet/venue needs to be somewhere people are actively going to in order to browse and buy handmade goods. There's no point in sticking handmade stuff on your personal social media page or FB marketplace.
B. Make your product more unique. If Primark sells similar acrylic or polyester (machine) knitted hats for less than a fiver, then yours HAVE TO BE DIFFERENT. And that's now very hard. The factories have cottoned onto the look of 'art' yarns so you would need to find something to knit with that looks very different and yet is still on trend or looks nice. Consider using natural fibres - you'll be able to big that up and Primark doesn't use actual wool in their crappy hats! Maybe learn to cable and add a cable detail? People seem to love pom poms on their hats (I can't fathom that myself, I think it makes grown women look like three year olds, but...). Self-striping/patterning yarns look more arty and cleverer than they are. Cute coloured ones for kids would possibly sell too.
B.II: please remember that it's your time, ultimately, that is your main cost. I knit socks (strictly for close family and the odd person I really love, as I'm a slow knitter and they can take ages). I would never, ever bother using a cheap synthetic sock yarn costing £5 because I know that a pair will take me at least a month (I knit while watching tv of an evening and have other distractions, hence the time!). Why do I want to spend so many hours of my time working with cheap substandard material? Why not spend more and buy a decent quality, high wool sock yarn (you do NOT want to know how much it's possible to spend on sock yarn... or maybe you already do)? Then after the investment in time, you have a decent product. Hats - you will hopefully cut the production time for them but all the same, think about the finished product. Buyers will STILL look at it and think - 100% acrylic hat, can buy it in Primark for 1/3 of the price and will NOT CARE that it took you two or three hours. If the same hat is made in nice sheeps wool, then it starts to look like a bargain and there are reasonably priced wool yarns out there.
C. Promote your product as handmade and (my advice) made with higher end yarn. If you have a stall, you're able to talk to people who are browsing and explain how long it takes, how long you've been knitting, what good quality materials they are (this really does matter). If you're online, you have the chance to explain why your item is nicer than a Primark hat in text. Use that chance.
D. Spread the word. Tell friends you have this cottage industry and ask them to tell their friends. Don't go mad making business cards but do think about presentation - I make tags for some of my things with brown kraft card and brown twine or baker's twine. Add a cheap stamp for decorative appeal/eye-catching (you can often find used scrapbooking stamps and pads in charity shops).
E. You'll need to have the odd weekend day free if you do events, markets etc. So make sure that can happen in terms of childcare.
F. Don't expect this to happen overnight. If people like your stuff, they WILL buy it and they'll come back (so make sure you have enough of a range of different ones, because a person will only buy the same hat once!). But it takes time to build up contacts, customers and get those spots at markets. And you're up against a lot of competition re. knitting or crochet. I bet there's a WI market locally. Pop in and see what they charge for hats, or a baby's matinee jacket. It's almost always a pittance, barely covering the costs of the yarn. I sometimes gently explain that this is really not helping other home-based makers out there who are trying to teach their market that handmade has an intrinsic value, but WI knitters don't care - they're doing it for different reasons to you.
All that being said, OP, I personally wouldn't try and knit for money (I'm too slow for a start!). People are lacking in imagination, as well as ready funds, and it's very hard to get the average Joe or Jane to appreciate the time, skills and materials that go into handmade items.