A salve of comfrey leaf and root, nettle and arnica would be okay to apply topically as long as there is no broken skin and care is taken not to get this on their hands/in their mouth. This salve should also only be applied short term (no longer than a week). There is some debate on whether comfrey is safe for children topically, but it seems to be okay in short term use.
For the arnica, it may be more safe to use homeopathic arnica for your child.
Other more safe options for growing pains (which seem to be more likely related to muscles after lots of activity during the day) are: cramp bark, calendula, St. John’s wort, California poppy, and oats (nettle is also very safe). These would help settle, reduce any muscle tension, sooth the nervous system and reduce some pain sensations and could all be taken internally or if you wish to make a topical preparation, most of these will also be nice that way.
If you have any further questions don’t hesitate to ask.