When i grow up i want to be an musician and dentist.
> When I grow up I want to be a musician or a dentist.

because i love ukulele and piano.
> I want to be a musician because I love to play the ukulele and the piano.

I'm very good at it, too.

> Correct!

I want to be an dentist too. because my teeth is very ugly and when i have some money, i help the poor childern.
> I want to be a dentist, too, because my teeth are very ugly and when I have some money, I will  help the poor children.

When i'm older, i wants go to space,travel the world,skydiving.
because i saw TV kim so yan is went to space and i want to see Earth.
> When I'm older, I want to go to space, travel the world, and skydiving because I saw Kim So Yan on TV  and he went to space. I want to see the Earth from space, too.

And i wants try skydiving too. because i want to see the sea animals,play with fish.
> Lastly, I  want to try skydiving, too, because I want to see the sea animals and play with fish.