The Saproling tokens aren't created until after Fists of Ironwood is on the battlefield, so you can't cast Fists of Ironwood on one of those tokens.
14-03-2017
wotc
If the target creature is an illegal target when Fists of Ironwood tries to resolve, Fists of Ironwood won't resolve and won't enter the battlefield. You won't create Saproling tokens.
14-03-2017
wotc
You create the Saproling tokens, even if Fists of Ironwood enters the battlefield attached to another player's creature.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.