5 years or not, you need to do what is best for you. As much as you may love your nanny family, they will, no doubt, put their needs first.
When I found out I was pregnant, I told my nanny family asap because I had a strong suspicion that mum was pregnant, too and even if she wasn't, I wanted to give them plenty of notice to find my replacement.
They found my replacement and she wanted to start far earlier than I had planned to leave so I was out and she was in. It was all very amicable and maybe I should have negotiated harder but ultimately, they did what suited them best. I had been with them four years.
I have looked after children, privately, for 19 years and I am yet to meet a family who would put the childcarer's needs before their own. Might sound harsh but it's true.
I would give notice sometime in July and agree with pp that There will be plenty of nannies looking for jobs to start in September. Also, are you in London? If I remember correctly, summertime could be a great time to recruit because many families are away so there will be less competition for your family 