So you thought the ending was fine. Good for you. Lots of people didn't. Their opinion is no less wrong then yours.
As far as the Child's Play thing goes. Hey, guess what? No one's holding a gun to your head and telling you have to donate. If you disagree, then don't donate. What? You feel guilty because of the children that aren't getting money because you like (or tolerate) the ending? Well, fortunately, you can donate money directly to Child's Play through their official website!
Guilt averted! Children helped! No need to support the "change the ending" drive!
...but I get the feeling you'd rather just sooner keep your money.
As far as "you can't criticize this! It's someone's creative work!". Bull. People make a living criticizing people's work. This very website does it on a daily basis. Now with something like a book or painting, there's nothing you can really do, except apologize that you felt it didn't live up to their expectations, learn the lesson for your next series, and tell your supporters you're doing so, so hopefully they'll decide to stick with you. However, this is a video game in the 2010's, that have the ability to be altered through the internet. Hell, BioWare charged for this extra "DownLoadable Content" on the first day. By all appearances, it would seem that having an alternate ending DLC would be pretty popular. They probably could even charge for it. Hell, some people said they'd pay for it. Now, some have said that would piss them off more. Bet you that they'll buy it anyway. If it delivers, they'll probably even forgive them for it.
Besides, movies have done alternate endings for YEARS. I mean, hell. They completely re-edited Highlander 2 into a "Renegade Version" that removed all references to stupid aliens, purely due to fan "bitching". I Am Legend added an alternate ending to the DVD when the producers and writers were stupid and put in an ending that was not only a huge departure from the book, but made absolutely no sense, all due to people complaining. Just a couple of examples.
Want an example outside of movies. Hal Fricking Jordan. Go watch the Big Picture and listen to Bob's history lesson about the Green Lantern comic. Not only was the character brought back to life, but was absolved of all the atrocities he committed. This was done by a HUGE fanbase drive to do so. And one writer finally listened. As Garrus said in Mass Effect 3, sometimes to get people to act, you've got to kick and scream until you get their attention.
The thing is, we're not talking about "Some game that came out". If this was the original Mass Effect, people would have just shrugged their shoulders, not supported it, and the series would have died a quiet death. This is a series of games that people have become invested in for the last five years. Have developed emotional connections to the characters with. Wanted to know more about the universe it took place in. It was their passion for the series and their support that saw the sequels made, and all these people wanted was an ending that felt complete and was dependent of their actions as BioWare had promised MANY times, or at the very least, would make sense!
But it is not out of hatred of BioWare that people are complaining (for the most part. It's not that they hate the game. On the contrary, its because of the sheer love for series for the reasons mentioned above. It could have been the Star Wars of this generation. Even more so. I mean, in Star Wars, you just got to watch Luke deal with situations he's dealt. In Mass Effect, you didn't just get attached to the character, you were the character. You got to make the decisions, whether for good, or for ill. And to have all the choices you made, all the benefits they gave, all the consequences you had to endure because of them, to have all that (to many people) taken away and made irrelevant in the last five minutes of an otherwise exceptional series, it should be understandable why people could be that upset.
"It's just a game!" I can hear. Why cry over a piece of music? It's just music. Why be excited when a kid wizard in a book gets the best of his nemesis? It's just a book. Why get upset when the villain in the TV series unexpectedly gets the upper hand of the hero? It's just a TV show. Why be passionate about any form of medium? Should just devour it unemotionally, no matter how trite it is. Should we not demand better?
I'm not trying to convince you that the ending is bad. If you find it tolerable, again, good for you. However, for people to dismiss the ones that feel that the ending did not live up to the series they are passionate about as bitching, or even self-entitled, I find these people at best, self-entitled themselves, and at worst, hypocritical.
This has gone far beyond tl;dr. That's my two cents. Take it or don't. I don't give a damn if you didn't ask for it.