My first thought was that they will be chewed, so as everyone else already said, they need to be sewn on *very* well, but I would also stick to simple cotton fabric rather than anything fuzzy/furry/minky style. I personally wouldn't want my child ingesting fuzz (or at least, as little fuzz as possible - they will eat anything/everything. Unless you're at the weaning stage and it's actually food.)
And congratulations on the anticipated granddaughters!
