Do you think what we did was mean? I thought we were teaching my younger cousins a lesson?

My 8 and 5 year old cousins were making prank phone calls last week and my older cousin (their mom) found out. So she asked me and my younger sister to "help her something." Our cousin said we should prank them to teach them a lesson. So she had me call the 8 year old's phone (check phone) and I made up something like they won a trip to Lego Land and they sounded happy. Then later we revealed it was us that tricked them. My younger cousins were upset and said, "That wasn't very nice to trick us like that!" Then their mom, "Well neither is pranking other people. Know you know how they feel."
They apologized and then their mom said, "Okay then. NO MORE!" With a stern tone. She had to call some people and explain everything. My sister thought it was a good way for them to teach them a lesson. I thought so too. My friend said that was harsh and mean and they're just little kids. I told her, "Well that's the only way to get them to stop. They haven't done it since." My cousin is a very firm and stern mother but loving.
Am I right?