Pretty good voice.
The only thing I never liked about it was that it always sounded "held back". There's many other entertainers, like Billy Idol, who have a pretty solid voice but it never sounds open or powerful.
Her singing style is on purpose and I'm not sure why they made the change. I never question her singing ability because at a young age she could belt and had power. They are purposely going for a younger sound childlike sound.
Britney at ten years old.
Both Britney and Christina have proven their singing ability. What's unique about them is they are both great singers and great dancers.
The vocal trinity including Whitney, Mariah, a Celine are unmatched in their singing ability, but none of them dance. Their feet are planted on the floor and don't move for good reason.
The women who were first dancers, but got into singing include Madonna, Paula Abdul, Jennifer Lopez, and Shakira. They aren't particularly strong singers, but are good enough and when combined with their dancing ability create very powerful performances.
This is where Madonna was a trailblazer and innovator. There's a clip in her documentary Truth or dare where the camera does a closeup on her mid concert and she is drenched in sweat. All of the dancers in that documentary said they had to step up their game to keep up with her. Truth Or Dare is now on Netflix if anyone is interested. It give an inside look at who Madonna really is.
Janet started out as a singer, but later developed strong dancing ability.
I find it amusing that most pop fans can't discern who can sing and who can't. There are so many things they can do in a studio with things like auto tune and turning up background singers that they can make an average singer sound good. Paula Abdul and Jennifer Lopez are examples, but their strong dancing ability make up for it and they are worthy of respect.