I'd like to provide proof, but the only thing I can find is for cross-border primary school students under 11, which aren't what I know about.
First, it's easy enough, in my opinion, for children under 11 with a pictureless HKID (that's all HK children under 11) to enter and exit HK in a manual line for HK residents with their pictureless HKID and their HKSAR passport (or other passport I guess).
With that noted, if you have children aged 7 to 10 with a pictureless HKID then feel free to ask enough immigration officials near an HK eChannel if there is a certain channel or two your child can use with only using their HKID. I recommend helping your children and letting them go right in front of you, so you can help them as needed.
Anyway, if you want your 1.1+m child under 11 to use an eChannel or enter or exit HK feel free to ask and try, and it 100% has worked for some non-cross-boundary primary school students under the age of 11. I can't find evidence of this being true published online by government sources, but feel free if you have a certain age child of yours try it with you, and then tell me I'm wrong.
This is the closest thing I can find: https://www.immd.gov.hk/eng/services...residents.html
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