Firstly, on behalf of my nation, may I just say KEEP HER AWAY FROM US!!!!!
Thank you.
Britney's a good entertainer, but her shows revolve around lip-synching (god did my ex hate it when I pointed out the half the time her lips weren't even moving! He couldn't take his attention away from the oh-so-obvious miming for the rest of the show. Rather spoilt it for him

) and the grand spectacle, backed up by a crowd of dancers.
Ok, so maybe she'd cope with TONIGHT'S THE NIGHT, then.
Btw, there's several reason why superb child singers can sound appalling as adults:
1) They overdo it as children - their voices are so remarkable for their ages that they are pushed too far. People believe that because they have voices beyond their years these voices are fully mature, which of course is not the case (prime example: Charlotte Church. Example in waiting: The Labbadia sisters).
2) Their technique is poor, but this isn't noticed - usually because they sound naturally good so the parents think there's no point in them having singing lessons because they don't need it.
3) As they get older their voices simply change. Plus, they are older. Remember, a lot of child singers are remarkable for being able to sing like that AT THEIR AGE. They may possess voices just as strong, or with the same quality of tone, 10 years later, but they are then being compared to adults, who (through training) may be far better at using their voices and controlling breathing, tone, and power. The benchmarks change.