OFF TOPIC answer!
IMO, arnica is incredible. I'm pretty careful with my homeopathics, and rarely just randomly use a remedy, but arnica, I could write an ode to it.
The Boiron brand stuff is good enough, but last summer DH and I were moving from house to apartment. We were, especially Robert (DH), extremely sore. Our apartment is near downtown and we walked a few blocks to a farmer's market (downtown, odd!), and passed by a booth where their arnica cream caught our eye. We took a tiny sample of it and I smoothed it on DH's calves, and by the time we were at the next booth, his calf muscles had stopped screaming at him. We went right back and bought the bottle.

In fact I just ordered a bottle to have sent to my stepdad who is having a hip replacement tomorrow, b/c he had a very hard recovery from the first side's replacement...I figure once it's OK to smooth anything on his skin near the incisions, he should smear that stuff, b/c arnica can stop pain AND it promotes healing. (in case anyone wonders, the company's name is Salmon Creek Botanicals, and they have a website)
So
I would feel comfortable giving my own 5 year old arnica, either pellets or using the cream/gel, for growing pains, especially since I use one or the other on my 3 year old's growing pains, headaches (he inherited leg pain from me and head pain from DH, poor kid), and various bumps and bruises, and his pain stops VERY fast and he heals up quickly.
But homeopathy, while not dangerous, can be tricky in some ways (if you over use something you can make yourself "immune" to it...we did that with chamomile when DS was teething, when he ate a whole bottle of pellets...took a year for it to work on him again), and even though
I use arnica on myself and my family without any worries, it always behooves people to look into it if they want to!
So that's an answer and an addendum to my above mention of arnica, lol.