Go to www.hylands.com. Go to products and look at Calms Forte 4 Kids. You can buy it at almost any store. Walgreens, WalMart, Kroger, Target, etc. I used their teething tablets as well. I love how safe all their products are. Here is some info on the Calms Forte that I've copied off the website:
Watching your restless child toss like a tiny boat in rough seas, you’d give anything to bring on the calm. These sleepless nights are heartbreaking as much as they are exhausting – for both of you. But even the youngest human body has the ability to re-establish balance and health – including rejuvenating sleep patterns. The trick to achieving this balance lies in your ability to provide safe relief that targets the causes that keep kids up at night. Hyland’s created Calms Forté 4 Kids™ for just this reason.
This medicine is designed to safely and effectively counteract mental and physical restlessness in children. Studies show that 69% of our children under 10 years of age are prone to some kind of sleep problem. This doesn’t mean that your child needs to suffer. Calms Forté 4 Kids™ addresses the causes that inhibit a child from sleeping well. Your child won’t wake up groggy like he or she might, when given over-the-counter drugs containing diphenhydramine (such as Benadryl®). This is significant for school-age children who need to be alert first thing in the morning.
Like all of our homeopathic medicines, Calms Forté 4 Kids™ is a formulation of all natural, safe and effective ingredients. A calmative solution to restlessness, this medicine comes in easy-to-administer tablets that dissolve almost instantly in the mouth. Calms Forté 4 Kids™ works without side effects and won't interact with other medications.
125 quick-dissolving tablets, easy to administer
Formulated for children ages 2 and up
No side effects
Can be used with other medications without contraindications or drug interactions
Aspirin- and acetaminophen-free
Addresses other causes that can inhibit children from sleeping well, including night terrors, growing pains, and sleeplessness from vacation travel.
Useful for irritability and wakefulness
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If you don't like the Hylands, you can go to any pharmacy and ask about what dosage of Melatonin would be safe for a 21mth old. It's also an all natural supplement available anywhere. Good luck, I know that's very difficult to deal with! I hope it works for your daughter. =0)