My grandson's birthday

Thursday, it is Louis' birthday !

On Thursday, we'll celebrate Louis' birthday!

Or

It is Louis' birthday on Thursday!

Tomorrow i will buy him a present.

I will buy him a present tomorrow. 
He would like a bus fire or beads. He likes to make necklace for him sister and mother.

He would like a toy firetruck (?) or beads to make necklaces for his mother and sister. 
I will cook a chocolate king cakefor him.

I will bake a chocolate cake with a king on it for him.

Or do you mean...

I will bake a large chocolate cake for him.
He will blow the candles. Him sister will blow again.

He will blow out the candles and his sister will blow them out too/ out after.
Averyone will be happy.

Everybody will be happy.