Why not use a common sign language and give then a head start if they ever do want to use it outside the family?
They might not have the dexterity required for some of the more complex signs, I would guess. If you devise your own gestures they can be much simpler.
The signs chosen to teach to babies tend to be pretty simple. Things like "more" or "milk" are very easy.
No, the basics like hungry and please/thank you are fairly simple. The daycare my son goes to teaches all the kids sign language starting at like 6 months.
plus you can use it as a battle language for your clan
Exactly! Using a pre-made sign language is missing the point entirely...