The patterns that I have for playmats (and they specifically have 'playmat' as part of the name) are really just quilts with something of interest on the top...pockets, tabs, pictures, things to entertain the child. One has fasteners for the child to learn...zippers, buttons, laces, snaps, velcro, things like that. They aren't made any differently than standard quilts, except for one that has home dec weight cotton for the background fabric on top. The emphasis is on 'play'. Could be a quilt of streets for a little boy to use his trucks on, that kind of thing (didn't mean to be so stereotyping with that idea, sorry).
A lot depends on the age of the child, his/her interests, and what the mother has in mind, so I'd definitely ask for at least her general ideas.