It's possible that they are in dire financial straights. Your son broke something of theirs and you offered them money to make it right. They took the money, but instead of fixing a luxury item with the cash, decided to put it towards some credit card debt or buy groceries that month. Realizing that putting the money towards a luxury item like a trampoline would be irresponsible of them. So they fixed it up as best they could, willing to just do without it completely, because they needed the cash for other things. I would not begrudge them that.
You made amends for your son's wrongdoing and should be at peace with that.
We had a similar, but slightly different, problem at my house. My husband was out of work for a full year, then underemployed for another full year. And we're still in this boat. We were doing all right for a little while, making all our ends sort of meet.
My aunt sent us a check for $70 to pay for swim time at the local pool. (It costs money to get in.) The $70 would have let us get in about five times. She sent the money because she wanted us to have a little fun, knowing how tight our situation has been. I graciously accepted it and wrote her a letter of thanks.
That month my husband's hours were slashed by nearly half. We were going to have to put many of our bills on the credit card again that month, just to get by. Again. I had the $70 in my bank, waiting to be used for swim trips, but I paid the electric bill instead.
I never told my aunt. She thought we had fun times at the pool and it made her happy to keep thinking that.