My dad is a dirty hippy elementary school teacher, and we don't agree on much, but he's nailed some things.
Talk to your kids like crazy when they are babies. Got a 5mo in the shopping cart at grocery store?
"I'm pushing you in the shopping cart down the aisle at the grocery store so we can buy food to eat this week. I'm going to buy one box of cheerios cereal, in the yellow box, and one box of rice crispies in the blue box. We put it in a bowl, then we pour in milk, and sometimes honey for a yummy breakfast in the morning"
It's long winded as fuck, but they soak it all up like a sponge, and when they are ready to speak, they are good with words and shit
Doesn't necessarily make them any "smarter" but it gives them a jumpstart on vocabulary, which gives them a jumpstart on communication, jumpstart on learning, etc