B.C.
I would use baby shampoo because it is gentle and maybe try detangler spray? I wash my daughters Barbies hair and have washed Barbies from when I was a little girl ( so they would be 10 years old or older) with not much hair loss. The Barbie hair seemed to be rooted better when we were kids, I have noticed her Newer Barbies seem to lose hair just from brushing the hair...but then again what wasn't made better 10, 20, 30 years ago = ) Lots of luck! I am sure she will love playing with them.
B.