Sure! The sentence you wrote is mostly correct but can be a little clearer and friendlier for a 6-year-old. Here's a good way to say it:
"Make sure to pump the tyres so your bike ride is smoother and more fun!"
Let's break it down:
- Make sure to tells someone to remember to do something.
- pump the tyres means to fill the bike tyres with air using a pump.
- so your bike ride is smoother and more fun explains why you are pumping the tyres.
This way the sentence sounds friendly and explains why pumping the tyres is helpful!