Here is what I found on Google.
Singing lessons for children: When, what and how?
When I started out as a professional singing teacher, I did what a lot
of other singing teachers do: I didn’t take on children under a
certain age. If someone phoned and told me their 9-year-old daughter
wanted singing lessons, I used to say “no” straightaway and suggested
they’d rather go and look for a children’s choir or something similar.
I took this stance because children’s (and adolescent) voices are
usually not ready for the “full-on” training geared towards maturer
voices, a few notable exceptions aside. Most of us only reach adult
larynx proportions in our 20s – there is really no rush, and what
applies to learning other instruments (“the earlier, the better”) does
not necessarily apply to singing.
At some point I thought: Well, they will sing anyway, so maybe it is
better to “catch them early” and make sure they don’t develop any bad
habits. So a few years ago, I decided to reconsider my stance and now
rather take on young children instead of letting them shout and growl
along to their favourite tunes without any supervision at all.
At what age can my child begin voice lessons?
At what age can my child begin voice lessons?" This really is a hard
question to answer and each teacher will give you a different answer.
Many teachers prefer not to work with a child until his or her voice
has developed to a certain level of maturity.[...] The question for me is whether or not they can handle a 30 minute lesson. Many children under the age of 5 are not capable of handling a 30 minute voice lesson, even if they are capable of handling a 30 minute piano or violin lesson. [...] So if you're wondering how old your child should be before they start studying with me, ask yourself how long their attention span may be.