If things have always been amicable, and everyone is upset currently over this one situation, I don't think jumping to the CSA/CMS straight away is necessarily the right way to go.
How much lower is the amount? A significant amount? The price of the wasted tickets perhaps? Rightly or wrongly, maybe he thinks it's fair for you to pay for the wasted tickets if he is blaming you for it. Perhaps Christmas or new baby expenses have left him short this month and he'll make up the difference next month... Who knows?!
Have you tried asked him why the amount is lower this time? Ask him if it a one off adjustment. If not ask him if he is changing the amount and perhaps he could discussing this with you as you have to plan and budget ahead too. It is true that when he has another child to support the CMS calculation will adjust. But that baby has not been born yet.
Don't threaten with the CMS unless you believe you will be significantly better off going that route and intend to take it. It's better to try and keep it amicable if at all possible. You might find yourself worse off and not know if and when any payments may arrive otherwise.
From what you say, it seems to me that he made a promise to your dd that he shouldn't of. He is likely upset to have disappointed her and to have missed the oportunity to share an exciting event with her that she would no doubt have enjoyed.
DD is upset by what appears to her as you stopping her doing something she wanted, and her dad promised her she could. He appears to have pointed the finger at you for preventing it. Yes that was wrong. However we all say and do things we shouldn't when we are upset and angry. If this is different to how he usually behaves I think maybe there should be some allowances made all round on this occassion.
You have said that it is not acceptable to do this, hopefully he has taken that on board.
Your ex probably thinks you are being unreasonable over the tickets. Maybe he thinks you could of changed your plans to accomodate him for this special treat/event. After all, an opportunity came up for your dd to do something fun and exciting with him and he assumed you would be happy to change your plans to enable this to happen for your dd. In the same situation you might also hope he would change his plans if the situation was reversed. The difference being you would pronbably have checked before buying the tickets rather than assuming your ex had nothing planned or better to do.
Perhaps ask him to give you plenty of notice next time if he wants to change the previously agreed days. Or to discuss the options before he books tickets in future so you can both work together for dd's sake to find a date that works for you all.
I assume you explained that you already had plans, and that if he'd spoken to you before he bought the tickets you'd have been able to advise him of that so he could then perhaps have picked a different day to book the tickets. Bear in mind there may one day be something you want to do with dd on her dad's weekend so if you can sometimes be flexible it might be better for dd.
Sometimes it can be better to be rigid with the dats though, especially if you have an ex that messes you and dd about and cancels and tries to swap and change all the time. Maybe I'm wrong bit it doesn't sound like he is like that most of the time though...?
So just step back for a moment, count to ten, and then see if this really is such a massive issue that things now need to be set in stone by a court and/or the CMS. Or perhaps things can move back to being amicable again between you like they have been up 'til now.