oh my god! I'm surprised you've stuck with her as long as you have.
I'd just keep it really business-like - just turn up with a letter stating that you no longer require her services and are giving her whatever her contractual notice is.
Hand it over and say "we've decided to change childcare provider, so thank you for all your good work - here is our notice in writing."
If she asks why then you've got a choice:
- you can say that you can't cover the leave later in the year and have decided to change provider now in order to give your dc time to settle in - but you need to have a response ready if she offers to cancel the leave
- say that your work is no longer prepared to give you as much flexibility and you've decided to investigate different types of childcare (eg nursery/after school club/a childminder who works in a network) in order to ensure that there won't be an issue with staff holiday/sickness. Of course if you do choose another solo childminder that could be awkward if you think she might find out.
Good luck - don't worry too much though - people change childcare provider all the time and it rarely causes bad feeling.