The keys are not that difficult to find in this game but maybe you have trouble to find all of them.
Or maybe you are just to lazy 
World 1
1-1
1-2
1-3
1-4
1-5
1-2
1-3
1-4
1-5
World 2
2-1
2-2
2-3
2-4
2-5
its blocked by a wall but inside there is a bomb moster thing, just wait or spam tail whip
2-2
2-3
2-4
2-5
its blocked by a wall but inside there is a bomb moster thing, just wait or spam tail whip
World 3
3-1
3-2
3-3
3-4
on the jump section with the icecubes jump left instead of right
3-5
3-2
3-3
3-4
on the jump section with the icecubes jump left instead of right
3-5
World 4
4-1
at the portal section just fall down with the platform
4-2
4-3
4-4
bevor you dropp down to the platform with the lever, stay on the platform above it
4-5
after the section with the red, green, red blocks stay on ceiling and jump over the gravity switch
at the portal section just fall down with the platform
4-2
4-3
4-4
bevor you dropp down to the platform with the lever, stay on the platform above it
4-5
after the section with the red, green, red blocks stay on ceiling and jump over the gravity switch
Now you have access to the bonus level.
Gongratulation 
By Aida202
