I think a slightly *bigger* afghan (that is, if you normally do a baby blanket) would be appropriate for a 19-month-old.
;-)
also, a 19-month-old might have a favorite color, so you might inquire is this is known (or if it exists yet).
What a nice idea! And even if the child ends up not staying with them, it'll be a nice memory from that time in his life.