I disagree that a nanny will always put her child first - often her child will be last! Most of the time the child with the greatest need will come first.
What will you do when she is on ML?
Does she want to come back?
Does she want to bring the baby?
Would it be every day or will her partner/parent/mil/whatever have the baby for any of the time?
Is she prepared to come back for the hours you need?
You'll need to agree on what happens when any of the children are ill.
You don't have to let her bring the baby back with her - if you do, it will be a new contract when she comes back & you can have a trial period and even if you don't the 1 year thing will still apply anyway.
How many kids do you have?
How old are they?
You need to consider things like baby/toddler groups, swimming, etc What would she want to take her baby to - would yours get anything out of it or would they be bored? Are there things that yours & hers would both be able to go to?
A friend of mine has a 9yo and a 6yo - their nanny has been with them since the eldest was a baby. The nanny has a 3 year old boy - he's like their little brother. They are all really sad as now the nanny is pregnant again they have decided it's not going to work anymore :( but until now it has been brilliant.
Another friend has a 4yo girl and a 10 month old baby, the nanny has a 2 year old girl and it's all been good so far, but the nanny is pregnant again and they have decided that wont work anymore either, which they are all a bit sad about, but this has also worked really well for 3 years.
It's a minefield but if she's a great nanny it's one worth walking through.