I think Yes to Cucumbers has pretty good slip. And it's around 18oz maybe? I can't remember if you can buy it at Wal-Mart or Wal-greens though. Target for sure.
I don't understand why they are so stubborn. I have a ton of hair and had it that long when I was a kid. I finally got tired of it but didn't want a drastic cut so my mom would cut off a couple inches every couple weeks so it would gradually get shorter.
Also, your niece is 10 so she should be taking some responsibility for her own hair. At that age, I was taking showers, washing and conditioning my own hair and brushing (I know, the horror!) it afterwards. Sometimes my mom would help with tangles but I didn't have many.
Coarse texture, normal-high porosity, normal elasticity (Komaza Care), very weak waves
cleanser: Redken Fresh Curls Shampoo, CJ Daily Fix
rinse out: CJ Repair Me, CJ Curl Rehab
leave-in: SM Milk, Oyin Hair Dew
jellies/creams: UFD's CM, CJ CCCC
gels: CJ CQ, BRHG, CJ PP