Start off by talking about what kind of 'hands' different animals have & how they use them.
How are people's hands different?
We have thumbs that make it possible to grip things, turn things, etc.
Then have each girl fold her thumb across her palm & tape it in place. (Adhesive tape works best.)
Pretend to be thumbless animals as you explore an outdoor area trying to catch things, lift up rocks, etc.
Or do it indoors - try writing, throwing a ball, drinking out of a cup.