Age Eight has much more in common with Age Six and Seven than it does with Age Nine and Ten. Eights are still basically “little kids” who cry easily and are genuinely frightened by scary movies, and things that go bump, and disruptions to the daily schedule, whereas Nines and Tens are much tougher, don’t scare or upset as easily, and feel quite worldly and self-sufficient. Which they are, more or less.
So your niece’s tears are proportionately less serious than a comparable amount of upset would be, coming from, say, a Twelve or Fourteen. Not to denigrate her suffering or anything, because her little heart truly is broken, but–it’s temporary. More temporary than you may realize. And yeah, “water park” and “ice cream” and “Harry Potter Five Again!” will go a long way towards fixing it. You wouldn’t insult a teenager or an adult by trying to fob them off with “dinner and a movie, and you’ll feel all better!” But this is an Eight–dinner and a movie will work just fine.
No, she won’t. Because by that time, she will have learned (the hard way, unfortunately) that her mom is not god, her mom is not her friend, her mom is not even a decent human being, unless some miracle occurs and the SIL suddenly steps up to the plate. There will be other missed birthdays and broken promises, and eventually, usually by age 10 or 11, the kid involved gets a clue and grows a callus.
My BIL was a similarly worthless piece of shit who sired a beautiful nephew for us with his third wife. About the time Nevvie hit school age, BIL’s third marriage went the way of his first and second marriages, victims of his apparently congenital inability to follow through on any project he’s ever started (fixing porches, getting married, engendering children–doesn’t matter, he always, invariably, quits halfway through).
IOW, he split. Couldn’t do it any more. Boring.
And he took up the exact role of your SIL–the Good Guy, the Present-Bringer, and ultimately, the Promise-Breaker. Over and over and over again.
Finally, one day, when Nevvie was 14, even though his visitation rights called for him to not take the kid out of the state (Georgia, in this case), he took off to Alaska with Nevvie–and with Grandma, who has always been BIL’s enabler–and induced Nevvie to phone home and tell Mom they weren’t in Alaska. Well, she’s not stupid, she got the truth out of her giggling son in about 10 seconds, and she called the law on BIL, with the result that he never saw Nevvie again until the kid was 18 and left for college in Florida.
Anyway, the point is that Nevvie had no trouble at all understanding, after about age 12, that Dad was not going to follow through on anything he promised, and developed a tolerant, resilient attitude towards the whole thing. And after the Alaska dustup, he obtained a new insight into his dad, and any remaining vestige of respect (or emulation) went right out the window.
So your niece will come to that point, too. Just keep on not poor-mouthing Mom to her, preserve a tactful silence, and she’ll eventually figure it out for herself.