You can optimize for being smart, you can optimize for seeming smart, but sometimes you need to pick which one to optimize for.

It's important to note that, sometimes, seeming smart can be the right choice. If you need people to trust you in the future on a very important issue, you might want to always seem smart. It's kind of Dark Arts, though.

It may make you uncomfortable to admit to not knowing something. It may make you feel like the people around you will stop thinking you're all-knowing. But if you don't know how to ask stupid questions, and you just keep pretending to understand, you'll fall behind and eventually be outed as being really, really stupid, instead of just pretty normal. Which sounds worse?

This is mainly a social issue. You can "solve" it in your social environment by pointing out that asking questions is indeed the smart move. If you indeed are smart, the quality of your questions in the long term will be a sufficient proof of it.

P.S. Note that I'm answering ignoring the "10 years old" part: I don't think I'm sufficiently qualified to answer it, but the issue is still relevant for grown ups.