This situation exists b/c of HIS choices and decisions NOT yours.
And this is only the beginning -- yet he is already trying to guilt and manipulate you...do NOT allow him to do this to you -- b/c it doesn't bode well for the long term unless you set boundaries with him immediately.
I'm sure you'll be arranging to take the kids to the fireworks yourself and they will enjoy that too. And it is probably best for all in the long run -- that you attempt to stick to the schedule and arrangement.
Otherwise your lives will turn into chaos -- with constant adjustments and no way to plan ahead which will lead to much resentment and problems all around.
Yes, there will most definitely be times when flexibility will be needed by BOTH sides -- but this is the 1st holiday event after the divorce and perhaps it is wiser to start to implement the plan in place.
And your xH needs to understand and start living the consequences of HIS choices.
This is his new life...he'd better get used to it.
