Pretending not to hear or running away is an old trick and is a very quick way for children to demonstrate getting out of doing something they may not particularly want to do or just showing signs of defiance, if not addressed it does get worse and you will find yourself as parent or co-carer that you are constantly repeating yourself and we all know what that feels like…yes very frustrating! The key to improving this behaviour is to be face to face when asking your child the task you need at hand for them to do. Keeping calm and not shouting through walls and rewarding with immediate gratification the task done works extremely well. This response from you will encourage your child to continue with the positive behaviour exhibited by them and in turn they start to focus on that rather than the opposite.
For moments when out in public gently give your children verbal notice in a conversational manner i.e.: That in 10 mins you will be leaving the park so that they know its coming up this is very good for children who are stronger willed and need to know the routine via every minute.
Remember this is only bad behaviour traits that can be broken and reformed quickly so be patience as you will see results quickly.