Kerry and Belle, I would suggest that you contact Lynn and ask about moving up as the grade levels are not cast in stone. She may consider it, but it depends on the class and she may be less flexible for the weeklong summer classes. If your child has a specific interest or aptitude in a subject she is willing to try to work it out, but some of the activities are not just academic and need other age-appropriate skills such as advanced fine motor skill, safety awareness, etc. The specific example she has described is that she would not be likely to let anyone move up to any of the dissection classes due to the fine motor precision necessary. Rocketry would likely be another I would guess for safety reasons.
Also she wants to make sure that she has enough spots for students in those age ranges. We have been quasi-regulars for about a year and she knows my DD7-almost 8 (in 3rd grade). So Lynn was willing to register her for one of the 4th-6th grade classes in April, but she was very upfront about the fact that if the class filled past capacity that she would likely bump us to our second choice the BER (K-3).
I know that Lynn works nearly all year on getting the summer classes put together. The remote locations and finding teachers are the hardest from what she has said. She is always looking for new ideas for classes, so I'm sure she would be receptive to suggestions.
In my experience Lynn is very accessible via email and will also speak with you by phone; she can be quite busy on the actual Hatterdays, so that is not usually a good time to get into any extended discussion. She is also a parent of a (former) HATS student so is sympathetic to these kinds of situations and is willing to work things out where she can.