Welcome to the Disney Parks Moms Panel, Taylor. I hope you're having a magical week.
I'm sorry to hear your niece isn't a fan of her stroller. Because I'm not quite sure how your niece's pink car looks like based on your description, it's unclear whether it is permitted. However, there is a list of
prohibited items found in the Disneyland Resort park rules. Guests may not bring into the Parks:
"Wheeled mobility devices with less than 3 wheels or devices that cannot maintain stability and balance when stopped, unpowered or unoccupied. Devices must be manually or electrically powered and operated at a walking pace. Devices should be single rider and not exceed 36" (92cm) in width and 52" (132cm) in length."
If you have further questions or concerns, you can contact Guest Information at (714) 781-INFO prior to your arrival just to be sure.
Sending you Pixie Dust,
Kiki