I'm really sorry this happened to you. Like Judy said, never chase a client. No matter how long you are in this game it will still happen to you. It's on the parents - not you!
I know it's frustrating to have an empty space you thought would be full but these nervous nellies probably would have made your life miserable.
As for the highchair, it's now your's for all future use, you can write off the full amount and the $150 deposit should cover it.
You don't want to get walked all over but you also don't want to reputation of being litigious. 1 mouthy parent can poison your whole well of potential clients.
The insult stings, the loss of income stings but there are times when it's just better to walk away and regroup.