There’s one thing I know about kids: if you’re sold on something, they’re more likely to buy it.
So prepare them by talking about how fun getting their picture will be; let them pick out a favorite toy or two; let them think of all the ways they can use their photo (send it to grand-parents and family; frame it at home; turn it into a mousepad, etc) Because if you’re excited, they’ll get excited too.
Then prepare yourself by having no set-in-concrete expectations.
Let the children be themselves! They can move or wiggle; be a little shy or giggle; make silly faces or jump around. They don’t need to smile on command or be told “Stop making that face!” or “Sit down!” etc. All that over direction kinda dampens their natural spontaneity and joi de vivre.
Babies don’t need posing at all.
And for babies, they’re always happiest to lie in their favorite position, whatever that is. Some babies have a favorite side, some like to stretch out on their tummy, some have unique little “power poses” that they like to do. But whatever it is, it’s natural and normal and beautiful without any need for props or fake “scenery”.
The main thing is to allow their uniqueness to come through.
And really, isn’t the point of capturing them when they’re little is to save just a little bit of their “little-ness” that you can later look back on and enjoy?