I'm happy that you have asked me to assist you with your child and hope that you can benefit from my many years of helping parents like you.
I checked to be sure but confirmed that Valerian is NOT to be given to children under 3. Here is the link to the National Institutes of Health article:
Since your daughter just turned 3 I would still have some reservations about using it. If you do use it you have to realize that there is no data to recommend what the dose would be so you'd just have to reduce the adult dose and see how he does. I'm sorry I can't give you a dose and I'm sure a Google search will result in some recommendations but you have to realize that there is NO data to support any dose recommendation for adults or children. Melatonin is a natural hormone that is secreted by the pineal gland in the brain to regulate our diurnal sleep/wake cycle. It also is not recommended due to the other effects this hormone has on sexual maturation which, though probably minor, is not to be ignored.
I'm sorry not to be more help or encouraging but of the two I'd probably try a reduced dose of valerian to see how that helps since you've already been through melatonin.
Please let me know if you have any further questions.
Thanks - do you think that Melatonin is bad for him (I have the dissolvable strips and tear of a tiny corner for him). It always works (even though it is such a small piece) but I just worry about the long-term effect it may have on him. Otherwise which form of Valerian is best for children - drops or capsules and how much could should he have please?
Would you advise as to whether it would be okay to, very occasionally, give him a tiny bit of melatonin in the afternoon to make him have an afternoon sleep please? Or do you think it should only be given at night time when we are desperate?! If he has had an afternoon sleep, he generally sleeps better at night time!