Uh no. It matters. If you tell someone you're going to be somewhere at a specific time, then it is rude to not be there at that time. The only way to make it okay is to call ahead and warn them that you will be late.

Even with a playdate like you said, the other parent may have food ready. They may have declined a different activity so they could meet with you. They may have left early from a different event, or even they were enjoying a TV program but stopped early to go meet with you. All of which if you would have called to say you'll be late they could have continued. It doesn't matter that you were meeting at a park, you put their life on hold for it, respect that they did that for you.

You have kids. Learn how to be on time, you're supposed to be the one who can manage the time for them, not use them as an excuse.