As a dependant of a UK citizen (as you and your daughter would be) you're entitled to NHS treatment. Just register with a GP when you arrive in country.
The 6 months guideline is for the minimum amount of time that must be REMAINING on your passport (Adult passports are issued for 10 years, child passports are usually issued for 5 years depending on the age of the child), not how you have to physically have them before you can apply.