Ads are literally everywhere. If it were easy or possible to prevent kids from seeing ads, then I'd be able to avoid ads myself, but that's basically impossible. Nearly all manufactured products have logos on them, and that's literally just the beginning.

It's not like OP was saying he kept his children deliberately unaware of what ads are, he just limited exposure as much as possible. I'm sure if OP's kid saw an ad, he would explain it to them / advertiser's motives.

But the idea that "more exposure" to ads makes you somehow more resilient to them is the most ridiculous thing I've ever heard. Does Facebook make people more well-adjusted? Does instagram make people less inclined to be shallow? Does increased time on Twitter make people less vitriolic?

When digital networks have an adverse effect on the human psyche (mainly ad networks and social networks), the solution is very demonstrably not "just watch more and you'll become inoculated against them".