I don't think it's typical, but it is widespread that agents will commit to a verbal agreement pending work or a success path with a developmental child client.
Some kids never get launched and are out by the third audition because they lose interest or realize it's not as fun as it sounded to begin with!
I think that many agents will test the waters. Signing clients is time intensive.
What's the down side? That the agent shows a mediocre level of interest? Possibly.
More likely they just want to see that the child is excited and committed to doing well.
For me at CESD, we want our talent signed and want to show them our committment. I tend to sign from age 6 and up.
Hope that helps!
Carol Lynn