Think of singing lessons as learning to play a new instrument, like piano, except that the voice is actually a combination multi-instrument, so its a bit more complicated to learn.
If your child:
Is motivated to sing
Can concentrate for 30 minutes
Is committed to practice at home at least twice a week (starting with 15 minutes per practice) to vocal exercises and scales
Is mature enough to commit to long-term lessons
Has patience to work through singing barriers and see them through
Is between age 7-9 (or displays all of the above)
Then they could be ready for singing lessons.
A typical lesson could consist of:
Technique instruction
Learning about the voice
Scale exercises
Singing songs
Recording the voice at appropriate intervals, etc.
As their voices are still developing, consistent repetition is necessary to ensure correct, healthy technique is put in place as well as muscle memory and strengthening the voice. This takes time and most children need to take long-term singing lessons to reach a good level of singing.
Singing exercises and scales (vocal development apart from singing songs) can be boring for some children who "just want to sing" and singing lessons may not be appropriate for them yet.
Young children will need some help from parents to get into a good practice routine at home and may need some supervision during practice.
I encourage parents to attend lessons with their children so that they are aware of what they should be practicing at home.
"Singing is an amazing journey of finding your voice."