Your sentence is almost correct but can be improved a little to sound better. Here's the original sentence:
Penguins are slow walkers and when they want to go fast, they slide on their tummies.
Here's a corrected version with better punctuation:
Penguins are slow walkers, and when they want to go fast, they slide on their tummies.
Explanation:
- You need a comma after "walkers" because it separates two ideas joined by the word "and."
- Everything else is good and easy to understand for a young child.
So, the corrected sentence reads smoothly and is grammatically correct!