Check the battery and make sure it's not getting too hot. Otherwise, charge away.
A neighbor's house burned down about a year ago - after a UL listed factory charger that came with one of those kids battery driven jeeps somehow failed. I've been a little worried about leaving laptops, phones, motorcycles etc. on chargers since. You always hear about those one in million batteries that blow up. ???
They just moved back into their rebuilt house about 6 months ago...