My child is so excited by his day at school that It is hard to get him to go to sleep at his bedtime.
This answer complements the teacher as they have clearly enthused your child at preschool. The second part of the answer explains to the teacher that you the parent find it hard to get your child off to sleep as they are still excited and thinking about their day at preschool.
After coming home from preschool, he/ she is happy all day afterward.
After coming home from preschool, he/ she is happy for the rest of the day.
"After coming home from preschool, he/ she is happy all day afterward."
Until he/she goes to sleep is the rest of their day so you can say 'all day afterward'.
"After coming home from preschool, he/ she is happy for the rest of the day."
'For the rest of the day' means for the remainder of the day.
"After coming home from preschool, he/she is happy all day afterward."
（プレスクールから帰宅した後はずっと機嫌が良い。）
お子さんが寝るまでの時間を"all day afterward"（その後ずっと）と表せます。
"After coming home from preschool, he/she is happy for the rest of the day."
（プレスクールから帰ると、その後はずっと機嫌が良い。）
”For the rest of the day"とは「一日の残りの時間」という意味です。
On the days that my child goes to preschool, he/she is in a good mood up until he/she goes to bed.
My child is always in a better mood after he/she returns from preschool.
On the days that my child goes to preschool, he/she is in a good mood up until he/she goes to bed. - since it's not everyday that your child goes to preschool, you can use the expression "on the days" to express the specific days that your child 'does' go to preschool.
My child is always in a better mood after he/she returns from preschool. - you can use the word "better" to say that maybe your child is in a bad mood in the morning but then after returning from preschool, he is in a good mood or more good (better) mood than the morning.
On the days that my child goes to preschool, he/she is in a good mood up until he/she goes to bed.（うちの子はプリスクールに行った日は寝るまで機嫌がいいです）
→お子さんは毎日プレスクールに行っているわけではないということなので、お子さんがプレスクールに行った日のことは、"on the days" という表現で表せます。
My child is always in a better mood after he/she returns from preschool.（うちの子はプレスクールから戻ってくるといつも機嫌が良くなっています）
→"better" を使うと、あるいは朝機嫌の悪かった子どもがプレスクールから戻ると機嫌が良くなっていることを表せます。
He/she is always in a good mood after coming home from preschool
He/she is happy all day after returning from preschool
If someone is 'happy' then they are also said to be in a 'good mood' the first sentence explains that your son/daughter is feeling happy when they get home from preschool
the second sentence suggests that your son/daughter is happy all day after coming home from preschool these are both a good way to tell the teacher your child is happy there and they are doing a good job
「機嫌がいい」は 'happy' に加えて、'in a good mood' でも表せます。
一つ目の文では、「（息子／娘は）保育園から帰ってくると機嫌がいい」と説明しています。
二つ目の文では、「（息子／娘は）保育園から帰ってくると一日機嫌がいい」と伝えています。
どちらの言い方でも、「先生（あなた）のおかげで子どもが保育園を楽しめている」と伝えることができます。
"He/she is always in a good mood after coming home from preschool. He/she's in a good mood until he/she goes to sleep!"
"My child is always happy when they come home from preschool, but then is moody when they go to sleep in the evening"
if you wanted to explain that when your child returns home from the preschool they attend in the morning, they return in a good mood, until they go to sleep, you could say any of the following: "He/she is always in a good mood after coming home from preschool. He/she's in a good mood until he/she goes to sleep!" or "My child is always happy when they come home from preschool, but then is moody when they go to sleep in the evening".
子供が午前中のプリスクールに出席し、寝るまで機嫌がいいことを伝えるには以下のように表現することができます。
"He/she is always in a good mood after coming home from preschool. He/she's in a good mood until he/she goes to sleep!"
（プリスクールから帰ってきたらいつも機嫌がいい。寝るまでずっと機嫌がいいんです。）
"My child is always happy when they come home from preschool, but then is moody when they go to sleep in the evening."
（私の子供はプリスクールから帰ってきたらいつも機嫌がいいけれど、夜寝るまで機嫌がいいんです。）
イギリス 
カナダ 
アメリカ合衆国 
