Caring for clams found in the Columbia river(Portland, Or)

Hello World,
I recently went to a beach on the Columbia river with my 7 year old
son. We were trying to make a sand castle when he came across several
small clams. He proclaimed himself the luckiest boy on the planet and
immediately asked if he could keep them. I, of course, answered yes. I
do not know how to care for these creatures. I've placed them in a
bowl of water and sand that I brought back with us from the Columbia
river. Is there any other things that I should consider. Thanks for
any advice.

-Dee