There is a wonderful product called "Natural Calm" that is a magnesium supplement. The majority of Americans are very deficient in magnesium. I take this at bedtime (they also make one for kids ;) There are other natural products including amino acids that help calm the nervous system. One of my favorites is GABA, which I also take at bedtime. Also, green tea is great because while it has some caffeine, it also has an amino acid "theanine" that also has a calming effect, so it counteracts the caffeine, which you really should avoid.
You also need to try to schedule downtime for yourself at least once a week - taking a bath in epsom salts (which is another good source of magnesium), along with a couple drops of an essential oil like lavender (which is also calming) works great.
You don't say how old your kids are or whether you are a SAHM. If you have younger kids and are at home with them all day, then you really should make sure to schedule some relax time for yourself when they nap - read a book and have some herb tea (chamomile is good for stress reduction).
Also, taking a high quality multivitamin and extra B vitamins and vitamin C can help tremendously. B vitamins are important for the nervous system as well as production of hormones like cortisol. Vitamin C is very supportive for the adrenal gland. And, of course, eating a healthy diet and avoiding sugar can help keep your blood sugar steady - for me it's the swings in blood sugar that can really trigger moodiness. Eating some high quality protein for meals and snacking on something like nuts or seeds can help a lot to maintain a steady blood sugar level.
Also, you should find an exercise that you enjoy - I cycle regularly and that is my big de-stressor - this can be done even if you have young children by putting them in a Burley - my youngest was riding in it when he was 5 weeks old (in his infant seat of course) and other kids should ride with a helmet (just as mom should). If I don't cycle for two days, it is much more difficult to deal with all the stress that is inherent in a mother's daily life and boy do I get moody. Research has shown that mothers are under more stress than soldiers on the front line on the battlefield. The Russians have used an herb called Rhodiola to help the soldiers. I have had fantastic results with that - it is an adrenal adaptogen, so it helps to keep your cortisol within normal levels versus the typical stress reaction that raises your cortisol and then the after effect is a big drop in cortisol, thus the inability to deal well with stress. Also, if you are low thyroid, there is an increased burden on your Adrenal gland and it is not at all uncommon for folks to end up with "Adrenal fatigue" which results in the inability to deal with much stress at all. So, getting all your hormones tested can be very helpful. You can do this on your own with a saliva test that you can order here:
www.canaryclub.org (this will give you a discount to the test offered by ZRT laboratories: www.zrtlab.com)
That canaryclub website also has some self-assessment questionnaires that can help figure out potential problems. An excellent book that I highly recommend if you want to understand more about all of this is: Feeling Fat, Fuzzy or Frazzled. It discusses how abnormal levels of the various hormones can result in these symptoms.