I was a full grown man when I did "Ups and Downs" with my niece. She loved it, but after three or fours lifting that child up and then down made my arms hurt and I had to quit (She would tehn insist on "More" and after a few minites I would oblige but again quit after a few "Ups and Downs").
I can see a full grown man or woman having the strength to do the number of "shakes" needed to do the damage seen in a "Shaking baby Syndrome" case, but a ten year old? Something else is going on here, what I do not know what, but something else.
Remember most 10 year olds are pre-Growth spurt children, quit small by standards of High School let alone adults. The average ten year old girl is 51 inches tall (4 foot, 3 inches) and 70.4 lbs in weight.
Average height and Weight of Children:
http://www.disabled-world.com/artman/publish/height-weight-teens.shtmlPlease note just because women can carry a 20-30 pound baby all day, does NOT mean they can "Shake" that baby. A woman's center of gravity in just below her naval (Belly-button) and thus when she holds a baby she holds it tight to her body so the center of gravity of the woman's upper body and the Baby's body is right above that center of Gravity of her entire body. Thus woman can carry a baby for hours. The issue is do a 10 year old girl have the upper body strength to shake that baby, which requires the baby to be held away from her body. That requires a lot more upper body strength then just carrying a baby and again something I doubt a ten year old can do to the extent needed to do enough damage for the baby to die of Shaken Baby Syndrome.