My initial reaction was "droolflute", but then I googled "musical instruments for babies" and came up with such websites as:

http://www.doe-a-deer.net/baby_musical_toys_
http://shop.nurturecenter.com/muinandtofor.html
http://www.amazon.co.uk/exec/obidos/tg/browse/-/1095926/203-1910176-4193558

not to mention:

http://www.bbc.co.uk/music/parents/yourchild/childdev_chart.shtml

YMMV