Unless she is a super science minded kid begging to do anatomy, I would hold off. Were it me, I wouldn’t be concerned about leaving Anatomy until 7th. I would look at doing something like this if I really wanted to keep them together as long a possible.
3rd — Z00 2
4th/K — Zoo 3
5th/1 — Astronomy
6th/2 — Botany
7th — Anatomy, 3rd — Zoo 1
Another option is to let your daughter choose either Astronomy or Botany for 5th/1st, and the following year she could do Anatomy and your son could do whichever she didn’t pick. There is also the Chem/Physics book which we’ve been enjoying. Depending on how you scheduled Astronomy & Botany – she may be able to complete the Chem portion in 5th and the Physics portion in 6th after the family study was done. I would not be afraid to let the Young Explorers series run into 7th, or even eighth depending on interest and abilities of your daughter. Not everyone has a bent for science, and unless you figure she is one of those highly motivated science lovers, I wouldn’t rush her. If she is a highly motivated science lover, she’ll push you.