You may not have to do any of that if you are actually using a childminder, most of whom are self employed (and usually use their own home and have multiple clients) and set their own hours, terms and holidays.
If you are taking on someone to be employed solely by you, in your home, working your timetable and so on, what you have there is a nanny rather than a childminder. And there is a lot more to it than just paying the PRSI, you need proper advice on being an employer. You need to register, maintain proper records, do all deductions from pay and file them, you need to understand all your obligations as an employee so holiday pay and rights, annual leave, sick leave/pay, all employee rights etc.....