Since nearly everywhere has hills, avoiding them is almost impossible. Even places that you could have sworn were flat as a pancake will reveal hidden inclines when viewed from the vantage point of a bicycle. Figure that any hill that looks big to you looks twice as big to a child. The key here is to get them up it fast by whatever means works best. Going down the other side is never a problem.
Children have an insatiable lust for speed.
To a certain extent the downhill ride on the other side can serve as sufficient motivation for getting your children up a hill, although eventually they will figure out that the time ratio is unequal. That means most of the time we're going up. Traffic can also cause a temporary interference with your bicycle pace as well as your enjoyment. Biking in traffic is never fun and a child's interest in it will lag as fast as yours. Only once in our experience has the presence of traffic provided the stimulant to an otherwise dull ride.