You're way overthinking this. If I say "it's takes surprisingly little" it's because I'm emphasizing that it does not take a lot and instead a small amount is sufficient.

Saying "it takes a surprisingly small number of emotionally stable adults to help kids " is emphasizing that it does not require a large number of stable adults and therefore a small number is sufficient. How many does it take? Surprisingly few. A few suffice. How else can you read this? You're surprised that a necessary number is small? But it's always necessary to have at least 0 stable adults and that's as small as possible - you can't get surprised that it's small! Instead you must be surprised that the sufficient number is small.