If my area is in any way typical, then there’s plenty of demand for term time only care. But this may vary from place to place, and it is very difficult for prospective CMs to do proper market research with any degree of accuracy. It is a little odd that teachers will invariably claim they do more working hours than the rest of humanity, yet need far less childcare, but this does mean there's a opportunity for your sort of plan.
You’ll need to consider what sort of income you expect/need to make it worthwhile, since your plan includes some factors which will seriously limit your earning potential.
The average CM earns around £10k pa. Taking into account all the necessary unpaid hours (admin, planning, prep, etc. conducted at evenings/weekends) very few CMs ever achieve anything like minimum wage for the total hours worked.
The most obvious limitation is that you need to eat, pay bills, etc. for twelve months of the year, but your clientele will be wanting to pay you for just nine of those months. You could feasibly charge a slightly higher rate for TTO care, or a holiday retainer fee, but these options are not without their downsides.
Your own children will count against your 'ratio' when present. That is, they’ll reduce the number of paying children you can have at any one time. A CM can care for six under-8yo's including up to three preschool children. You own children will reduce this number of paying attendees to five and three respectively.
Quite a few teachers these days work part time, and/or have some help with childcare from family. You need to decide if you can afford to take on part timers.
As previously mentioned, you’ll almost certainly need an earlier start. IME, teachers want to drop off by 0745 at the latest, and some as early as 0650. They usually pick up around 1600-1700. You may be asked to do some later evenings, due to school trips, parents' evenings or extra-curricula activities: occasionally, anything up to 2100 IME.
I doubt whether Ofsted would have a problem with your lack of "official" experience. Raising four children is more than enough real experience.