It's my understanding that the Amish are extreme pacifists and will not fight back at all. Also, I believe they only go to school until 8th grade so the boys he let go would indeed be little boys.
My guess is that if everyone had fought him together, maybe they could have overcome them, but with a pregnant woman and babies, they probably thought it would be better to comply. I'm sure they didn't expect he'd start slaughtering the girls.
Who would've thought that he'd do an exectution type slaying?
I'm not critising. You do the best you can. I'm just wondering if we need to have a different plan in the future though. How could they have done things differently? This probably never was even considered a possibility.