Your baby's primary teeth may be only a temporary tool for chomping, to be replaced during the early school years by his permanent pearls. But it's no less important to take good care of them now and to establish the habits that will lead him toward a lifetime of dental health. Decayed or lost baby teeth can interfere with good nutrition and speech development, and by not holding a proper place for permanent teeth, they can make the permanent ones come in crooked.

When Should I Start Brushing My Baby's Teeth?

Tooth brushing can begin as soon as baby’s first tooth pokes through the gums. Use a clean, damp washcloth, a gauze pad, or a finger brush to gently wipe clean the first teeth and the front of the tongue, after meals and at bedtime. Toothbrushes — moistened with water and no more than a rice-grain size smear of fluoride toothpaste — can also be used, but they should be very soft and with no more than three rows of bristles (a pediatric dentist or your pharmacist can help you find the finger brushes and a proper baby toothbrush). Toss any toothbrushes that have become rough at the edges (or that are more than two to four months old, because nasty mouth bacteria can begin to build up).

Should  I Brush My Baby's Gums?

Pediatric dentists recommend cleaning baby’s gums after feedings, which helps fight bacterial growth and promotes good oral health, long before baby’s first teeth start to appear. Rather than cleaning baby’s gums with a toothbrush, try a soft, damp cloth, or even a soft rubber or silicone finger brush, both gentle options with a nubby texture babies tend to love.

Can Babies Use Fluoride Toothpaste?

The American Academy of Pediatric Dentistry (AAPD) recommends using cavity-preventing fluoride toothpaste starting with baby’s very first tooth, rather than waiting until age 2 as was previously recommended. Use a rice-grain-sized smear of toothpaste for your baby or toddler, graduating to a pea-sized dollop by age three. And don’t worry if your baby swallows some of the toothpaste (as she almost certainly will)—in such a small quantity, it won’t cause any damage to the teeth. Starting in the second year, you can teach your toddler to spit after brushing.

Teaching Baby To Brush

Your older baby or toddler will probably want to try his hand at brushing himself; let him give it a go (if he has the dexterity and doesn't just get frustrated), but be sure to follow up with a more thorough cleaning of your own, using a finger brush or gauze pad, before bed. To encourage his interest in dental care, try a fun brush with a favorite character and bright color. And let him watch Mommy and Daddy take good care of their own teeth, so he learns that it's a habit to keep for life.

What to Do If Your Baby Hates Tooth-Brushing Time

Unfortunately, not every baby loves having her teeth cleaned—and when baby is teething and her gums are sore and tender, she might be especially resistant. Here’s what to try if tooth-brushing time becomes a struggle:

• Go easy: Baby’s gums are sensitive (even when she’s not teething), so if she really doesn’t seem to like the brush, try a soft washcloth and a gentle touch.

• Sing a song: For some babies, a little distraction is all it takes to make tooth-brushing palatable. Sing a favorite tune while cleaning baby’s teeth, or make up silly versions of standards (maybe “Old McDonald had a brush,” or “Mary had a little tooth”). Your little one may reward you with a tiny-toothed smile.

• Show her how it’s done: Seeing mom or dad brushing—and enjoying it—helps make a game out of tooth-brushing time. “Mom goes first...now your turn!”

• Let her play: Your baby will probably be curious about the toothbrush or finger brush. Encourage her interest by allowing her to hold the brush and examine it at her own pace. She may even end up putting the brush in her mouth, all on her own.

  source: https://www.whattoexpect.com/first-year/teething/brushing-baby-teeth.aspx