No, I don't think it's the same. If you explictedly state that your child is 3 and they choose not to charge you, then you get lucky and you are not breaking any rules, because they have said, ok, go ahead. It's liek at a restaurant, it's not stealing if they comp you for a meal.
If you walk by without saying anything, they are assuming that your child is under 3 and are respecting you enough not to stop and question you -- they are not assuming you did not pay for a ticket that you should have. Because someone is giving you the benefit of the doubt and assuming you will be honest, does not mean they are giving you permission to not pay.
We have roadside stands that sell flowers and vegetables. You are expected to put your money in the can, and take only what you pay for. Of course it is very easy to walk up, take something without paying, and leave -- perhaps flowers since they are only going to die in a few days anyway so they aren't really worth the money, and I have bought several things from this stand during the year....) and leave. Has the stand owner given me permission to do this because he hasn't stopped me? I don't think so. The sign giving the price of the flowers and the can for the money tell me what the right thing to do is. If I fdon't thinkt he flowers are worth it, I don't take them.
Now, if the stand owner is standing there and I mention that I am only going to buy the apples, but not the flowers since they are going to die ina few days, and he says, pay for the apples, and take the flowers. Enjoy and have a magical day -- then I would think that's ok and different from the above scenario.
And, morality aside, the more times people take the flowers from the farmer without paying, the higher the apples and flowers are going to cost down the road.