I normally on landing take out 100 QAR just as a backup, usually end up with it on the way back. Taxis's I always ask before I get in about card. Since your on creditcard, I assume its Mastercard or Visa, both networks are widely accepted.

Tipping isnt a clulture here and not requested by the bell hop or assistance given with luggage.

As for things to do, it depends on what your kids are into, maybe museum of illusions or theme parts. Trip advisor would have lots of good options for you.