I don't know that you could tell if a child has ichthyosis or not just from the activity levels. The skin doesn't develop the keratin layer until well into the 3rd trimester. In the womb, the skin is well hydrated and the child is kept warm by mom's insulation, so all of the problems they have after birth shouldn't play a role - especially in the case of EHK.

Nathan (affected) was a pretty quiet baby, but I've had friends that had very active boys and girls, so gender isn't reliably determined that way, either.

Have you done the genetics for your EHK? If you have, you could tell by doing amniocentesis.
Ichthyosis-en-Confetti Type 2
Husband, Nathan - 10, Elliot - 7, Oliver - 4, all affected.
I also have an unaffected daughter, age 8.

email: jennifer at confettiskin dot com
facebook - find me on "ichthyosis mommy spot" or "friends of ichthyosis"