Its legally a different thing. What it sounds like is neither here nor there.
You are wrong that 'young children' are detained under administrative detention. The youngest detained under administrative detention on record was 14. Thats a child, not a young child. Detainees are 16-18 and I think we can agree that a 16-18 year old should have some understanding of criminal/wrong behaviour and can also be manipulated by others in ways that makes their behaviour dangerous.
https://www.btselem.org/statistics/minors_in_custody
I don't know enough about the detail but Israel is not operating under what we would consider normal conditions in the face of constant and extreme terrorist threat. All countries suspend or amend their highest standards of legal process when dealing with terror threats. To be clear detaining children should be an absolute last resort and should be subject to strict oversight.
For some perspective the age of criminal responsibility in england is 10 and children of that can be held in secure centres here and on average over 400 children are in custody in this country. The age of consent for girls in gaza is 17 which suggests girls are recognised to be adults at 17.
Its a complicated subject but often coloured by some people's desire to paint Israel as exceptionally awful without any perspective or understanding of the issues.
Edited