Hyperlite used to make a 109cm, but from what I understand it has been discontinued. I searched for awhile, but finally settled on a LF 111cm for my daughter (who is only 2). I am having to modify the binding plates to get the stance close enough for her though. (if you check e-bay & craigslist you might be able to locate a 109)
My other thought was to buy a wakeskate (a 39" wakeskate = 99.06cm) & just drill holes for mounting bindings. I get that a wakeskate is NOT a wakeboard & such, but at this age, just finding the smallest thing possible is key.
Not sure the deal with the wake industry, because I can get snowboards down to 80cm (she has been snowboarding since 18months).