Edited to add: I think I would dilute most of these with water since it's for your 18mth old dc. Not sure how good undiluted essential oils are for baby's skin... Here are a bunch of recipes that I have saved over the years:
Easy insect repellent lotion:
15 drops of citronella essential oil, 15 drops of eucalyptus essential oil, 10 drops of patchouli essential oil, 8oz light unscented skin lotion. Mix well and apply to exposed skin.
Insect Repellent Oil:
8oz light oil of choice (such as grapeseed oil or sweet almond oil)
15 drops citronella essential oil
10 drops rosemary essential oil
10 drops lemongrass essential oil
10 drops catnip essential oil
Mix ingredients and store in a dark glass bottle (all essential oils should be stored this way). Smooth on exposed skin, repeat as necessary.
Infuse together in a canola or soybean oil:
1oz each of crushed rosemary, thyme, lemongrass, catnip, basil, bay leaf, and crushed cloves.
(To infuse, add 3 cups of oil to a pan or crockpot and simmer on low for about 1 hour. Let it sit for 24 hours and then strain out the herbs).
Apply on the arms, neck, and legs.
**Also great for horses! Rub around ears, neck, jaw, and legs.
10 - 25 drops of essential oil
2 Tablespoons of vegetable oil (olive oil is fine)
1 Tablespoon aloe vera gel (optional)
Combine ingredients in a glass jar, shake to blend. Dab a few drops on your skin or clothing. Shelf life is approximately 6 months.
Suggestions for essential oils to use -
Ticks (first choice): rose geranium. Other choices: palmerosa, bay, eucalyptus, lavendar.
Mosquitoes: pennyroyal, lemonbalm (citronella), thyme, lavendar.
Blackflies: sassafras, lavendar, eucalyptus, pennyroyal, cedar, lemonbalm (citronella), peppermint.
Head Lice: teatree, rosemary, lavendar, eucalyptus, rose geranium.
Fleas: orange oil.
Lemon balm or yarrow tea can be misted onto children to help repel mosquitoes.
Rub fresh basil, feverfew, or lemonbalm on the skin. Make a spray of water with citronella essential oil, or dilute oil and rub on skin.
Natural ingredients that repel insects -
Mosquitoes: basil, catnip, cedar, citronella, juniper, lemongrass, rosemary.
Flies: basil, citronella, juniper, palmarosa, rose geranium.
Ants: all varieties of mint.
*The American Chemical Society has released findings that shows catnip essential oil is more effective than DEET in repelling mosquitoes (about 10 times more effective)!