Perhaps an ad in the parish bulletin would be helpful. Plenty of young ladies will be happy to help.
For full time, an au pair will be probably less expensive than a hourly nanny. Au Pair are usually foreign young women and you will be required to provide room and board. The salary is about 150 - 200 per week.
A nanny salary is usually about 15 dollars / hour average. It can be higher depending on the number of children.
You can also try to find a mother's helper. These are usually very young ladies that would help the wife out for a few hours a week. It is probably the less expensive option.