If minder in your home, you must pay minimum wage at least. The only reason I can see why any childminder in your own home would suggest less than minimum wage is if the person in question would be happier with cash, ie black market. From my own dealings, it very hard to get any one for less than €10/12, esp considering youv two small babies, this would be a qualified, first aid, garda vetted person.
There is insurance issues as youv employed whats known as a domestic servant in your home. Also if your babies would be travelling in her car, check the policy personally for Class II. The childminder will also be entitled to holiday pay and bank holidays.
If its only 20 hrs a week, have you consider using a home based childminder, who would be responsible for her own insurance and taxes (if any due)....your local childcare committee has a list of registered minders in your area.