Questions about Arnica?

My two year old had a fall last night and has split his eyebrow open, we took him to hospital and he has had it glued shut, it is swolen and bruised but its actually a lot better than i expected, the wound isnt too deep but i'm concerned it might scar. Can anyone tell me if arnica is safe to use on a two year old and if i should get tablets rather than gel in case it affects the glue? Thanks!