Maybe a fast wide to mid zoom? Avoid UWA as far as possible as it is very easy to get distortion at the corner. For the candle blowing, if the lights are off and you want to capture the ambiance, do it with flash off but remember to pump up your ISO. Maybe you can do a trial beforehand so that you know what setting to use. Watch out for low shutter speed at it might introduce camera shake/motion blur if the birthday boy/girl moves when blowing the candles. Remember to reset the ISO when you're done.
do you have control over the kids?
tell them not to blow first cos you want to take their nice photo.
ask them to pretend blowing (blow softly).
make sure the naughty ones are relegated to the back of the groups.