Multiple periods in one booking

Hi,
We will be travelling in Japan in July and August with our family. My daughter is turning 12 during this trip. I understood that if her JR Pass is activated before she turns 12 that she can still have a pass with the child rate.

Therefore we also want to buy for her a 3 weeks pass and for the rest of the family a 2 weeks pass. That would mean that her pass needs to be activated 1 week before the passes of the rest of the family. Is that possible? And can we do this also on the basis of 1 joint order? Or does the entire order needs to be activated all in one time?

Can you please advice?

Best regards
Joost