i hv all 4 already and they are all out of place 

but i dun really bother so long as they dun make me feel pain
haahahah

omg r u serious? ><
tis is scary! 

Wisdom tooth as such don't cause any problems, only when they're impacted(doesn't come out fully into its 'normal' position) where they can cause problems and the most common is infection followed by pain, usually it is adviced that if your wisdom tooth has already been infected and there was a history of pain to get it removed due to the fact that the same problem will recur again and again.

Future infection yes, actual infection causing cancer is highly unlikely.
Most probably her permanent teeth are starting to come out and the primary/baby teeth are infront and will soon get loose and fall by itself by the time he/she is 6+ years old.
There's no correlation with age and getting wisdom tooth extracted, only if you have any underlying medical condition of which you should mention before undergoing and treatment in a dental clinic.

And the case of patient dying during wisdom tooth removal due to anesthetic complication is due to the fact that the patient did not reveal complete history and he consumed drugs that had severe reactions with the anesthesia, therefore it is important to reveal any underlying medical condition that you have/ any drugs/medications that you consumed.
Root canal treatment can be slightly uncomfortable/sensitive/painful during the first seating(assuming the dentist does a good job) and no more pain whatsoever on subsequent seating/appointments if it is a multiple seating root canal treatment.

There are absolutely no relation in between having dental caries/ infection with cancer.

Severe bacterial infection which can lead to other infections yes but not cancer.
I'm sorry for what you had to go through, but at the first sign of pain/infection it is best to remove the tooth as the problem would keep repeating.