The original sentence: 'The kestrel flaps its wings very fast so it can stay still in the air and spot a mouse.' is mostly correct but can be improved for clarity.
A better version would be: 'The kestrel flaps its wings very fast so it can stay still in the air and spot a mouse.' – This is actually the same as your sentence!
Why? Because it is already grammatically correct. Each part makes sense:
- 'The kestrel flaps its wings very fast' – Correct; it shows the kestrel moving its wings quickly.
- 'so it can stay still in the air' – This explains why it flaps fast: to hover.
- 'and spot a mouse.' – It describes what it wants to do while hovering.
So, your sentence is good. If you want to make it even clearer for young readers, you could say:
'The kestrel flaps its wings very fast to stay still in the air and look for a mouse.'
This uses 'to' instead of 'so it can' and 'look for' instead of 'spot,' which might be easier to understand.