Child cries when I make he practice!

I nanny for an 8 year old girl. Her mother put her in piano class a couple months ago and she seems to love it when I take her to her piano, except she cries when it's time to practice at home.

She goes once a week to piano class. Her mom waits until the day of piano to tell me she needs to practice. She's done this almost every week, makes up some excuse for why she couldn't practice with her. So it's up to me to practice with her and it's not fun, at all.

She yells at me, cries, demands she'll play only once and stares at me like she's going to kill me, seriously, murder me right there. I really have no idea why she does this. I always make it fun to practice or I'll ask her calmly, "Hey, let's practice some songs!" I've never made it to where it's a job to practice. She's 8, it's meant to be fun and plus I know nothing about piano so I can get on to her if she's playing the wrong note or anything like her teacher would, so it should be more fun to practice than go to class with someone correcting you the whole time.

My question is, should I recommend her mom take her out of piano? She seems to like it but hates to practice outside of class, I don't get it.