Tuning generally once a year is sufficient. Only professionals (and the schools where the piano is constantly in use) tune half yearly. So if your child practices 4 hours or more daily, then yes, tune half yearly, haha!
Heater must be on, it's to counter humidity.
If tuning is not carried out, then piano will slowly go out of tune, and when you do eventually get it tuned, the tuner may not be able to get it back to A @ 440 without breaking the string. Of course this takes some years before it does get to this state. And if a piano is deemed "untunable" then it doesn't have a resale value.