He realized his mistake when the children stopped visiting the garden and the garden never experienced the other seasons than winter.

The giant heard the mellifluous chirping of a bird when he was lying in his bed.

The snow-covered up the grass with its white cloak whereas the Frost painted all the trees silver

yes, god punish those who are cruel to children and those who are very selfish.

Here,

1. The selfish giant is the speaker.
2. He is speaking to himself.
3. He is referring to the children's who used to play in the garden
4. He is saying "the children's are the most beautiful flowers of all" as he realized the children are the soul of the garden.

Here,

1. "there " refers to the selfish giant's garden.

2. "they" refer to the children who used to play in the selfish giant's garden.

3. they are saying so as they are not allowed to enter and play inside the garden.