Not a dentist, but have two kids that both had cavities!
My daughter is 5, but had 4 when she was 3.5. They were between her teeth, touching each other. "Back in the day" I was taught kids didn't need to floss baby teeth...that it was bad for their gums. So, I took that as gospel, and didn't. She's not a candy eater, and we brushed regularly. I've been told by other mommies who nursed after the teeth started showing up, their kids had a few too. Mother's milk, falling asleep nursing, you get the idea. Didn't brush after those sessions, lol!
My son had one that was not so much a cavity, as a weak spot in his tooth that was ABOUT to become a cavity. The bonding material fell out of that one within 6mo and he had to have a redo. Not fun.
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No, you don't need a pediatric dentist. We went to one for my girl, because she's super sensitive, and the only difference was a sing-songy voice and videos while they do work. So now she can't watch that movie, because it reminds her of the dentist, lol!
Some fillings you really CAN'T let go, like molars stay in until they're 10 or 12 or something. It could rot into the adult teeth. But the 2 cavities on my daughter? Right in between the front teeth...and guess what I noticed was loose this week? Dunno if she could have lasted 18 months without the fillings, kwim? Besides I like that her smile stayed pretty.