You will only be required to pay class 4 NI if your annual profit is £7,225 or over... up to that amount you only need to pay class 2.
To earn a profit of over £7k you will probably have an income (before expenses) of around £18k ish... because childminder allowable expenses are sooo generous.
If you are taking all your expenses off your income you should - unless you are earning an absolute fortune - not have to pay much tax or any class 4 NI.
Having said that if you are earning that much before expenses you should have plenty coming in to be able to put 30% away a month so your tax and NI bills are paid when you get them.
If your accountant is getting it wrong you need to think about whether he is right for you... and either do it yourself as tutu suggests or find someone who specialises in working for childminders and understands our privileged position with the tax office.
I hope this helps!
