I really wouldn't go down this route.
I have friends who have done it, with older ladies also, and its been very hard for them and I don't think any of them would do it again.
It has been impossible for the children to have play dates, get invited to parties, be across everything that is going on at nursery and/or school etc etc. Therefore the language barrier isn't just a problem in emergency/one-off situations but it impacts the practicalities of every day life outside your home. So many of these things are organised informally at drop off / pick up. I think its really unfair on the children to miss out on so much because their nanny cant speak English.
Also, obviously, being that bit older means that she wont find it as easy to learn English that quickly. The younger you are, the quicker your brain works and so languages are easier to learn. In all likelihood, her English will probably never improve greatly if she is speaking spanish all day in her workplace.