Yes, it is permissible. Rabbi Neuwirth, Shmirat Shabbat Kehilchata 6:17, writes that even on Shabbat it is permissible to allow a child to ride a tricycle or to roller skate [blade] so long as the area is surrounded by a Kosher Eiruv. I think that it is important to point out that if a person lives in an area where it is not considered acceptable behavior to ride tricycles on Shabbat one should not allow ones children to do so.