Hi--sorry you felt stung by some of the other responses. I recently became a single Mom myself--so I feel you there, especially regarding the need to stay on budget!
I don't know whether you can change your hotel without a penalty (thereby freeing up some funds for the Halloween Party tickets) but reading that you have a four year old, I'm not sure that I would advise you to do that anyway.
I've only been to the Mickey's Not So Scary Halloween Party (at WDW) but since I went with my then four year old daughter (on Halloween weekend to boot) I'll share my thoughts on the value of buying an extra ticket on top of the hotel/park tickets you may already be committed to.
When we went, we had a WDW resort package that included park hoppers for each day of our stay and when I found out about the party, we bought tickets to it too. Because we had already paid for a day ticket (as part of our package), we felt compelled to spend the day in one park, then head over to MK that evening and stay at the party until the fireworks at the end (11pm that night!) It was a LONG day for us (especially for our 4 year old dd) and the next day we were wiped (but again didn't want to waste that day's park ticket)!
Plus, because it was Halloween weekend, the party was sold out (typical), lines were long (we spent an hour in line for one attraction--which if you break down the party ticket into a per hour amount that is $$), treat stations were swamped, and by the end of the night dd flaked out. (This was one of my early trips and clearly I suffered from being an over-achiever--wanting to get in everything possible LOL!)
In trying to decide what to do, one question I'd ask (and only you can answer this for you/your family) is: Is there something special about the Party (that you otherwise can't do) that you will be disappointed without? It sounds like fireworks are a concern--IF (and that's an IF) you can't see fireworks from inside the park any other night of your trip, then that is a factor weighing towards buying the ticket. (But even that isn't totally true--because there are locations from outside the park where you can get a view of the fireworks)
Another VERY important question is whether your group has the stamina to do the activities (that are already planned for that day of your stay, as well as manage on lesser sleep for the NEXT day's plans) given that you'll have a late night at the Halloween party. If your crowd is made up of night owls (who would not be tempted to arrive at a rope drop the next morning anyway!) and/or you can spend the day of the party (or at least a good chunk of the afternoon) resting/lounging poolside (without wasting an already paid for day ticket) that's a better fit.
Another factor for some people in deciding to go to the Halloween/Christmas parties is that they like to do something 'new' each time they go (often 'frequent flyer' DL types like to 'collect' new Disney experiences). It makes sense if you go often that it's a draw to see a new parade, new decorations, a new fireworks show, etc. However, the good news for people like us who haven't been frequent DL visitors--it's ALL new for us
Would we buy Halloween party tickets again? I wouldn't (at least not with young kids). Instead I would avoid DL the day of the party, go at rope drop to the non-party park (in this case DCA) and spend the day there. I'd plan to spend my day at DL on a non-party night (that way you don't lose any time in DL when day ticket visitors must leave due to the early closing of the park).
I hope that helps you in deciding what to do

Whatever your decision, I admire your strength in what has certainly been a trying personal time (your kids will benefit from having a Mom who overcomes the difficult circumstances in order to create happy memories with them!)
Have a fabulous trip--I can think of no better place to shake off a hard year (hence we'll be there right behind you in November LOL!)