I seriously don't get this.
The Welfare Requirements of the EYFS state that for three and four year old children there should be a minimum adult : child ratio of 1:8. (Similar rules for 2 year olds but I only take 3-4 y.o so I'm not that clued up on other ratios)
Of these adults, the room leader must be qualified to a minimum of NVQ level 3 in childcare, or an equivalent qualification.
Of the remaining adults, at least 50% of them must be qualified to a minimum of NVQ level 2, or equivalent.
So in short, if your preschool are not meeting these ratios and minimum qualification levels, they are in breach of Ofsted's rules.
They really shouldn't be relying on parent helpers to make up any shortfall here.
Parent helpers are always welcome and useful, but it's no substitute for planning your staffing properly. You can't always rely on parent helpers, and for good reason, e.g. sick children, domestic crises and so on.
Also (one more time, sorry, I know this is my pet subject), your child's free entitlement has to mean free. If it is costing you, be it in fines or in childcare costs, for your child to attend that preschool, then that is not allowed.
Ask to see their written policy about this.
Ask to see their Inclusivity Policy, which should refer to the steps they will take to make all parents welcome and able to access the preschool's services equally, irrespective of family background, culture, employment status, sexuality, etc etc etc.
If you still get nowhere, write to the Chair of the preschool outlining your concerns, and asking them to resolve the matter within 7 days or you will complain to Ofsted (they have to have a Complaints policy which tells you how to do this, too
)