How do the Cratchits react to their Christmas feast, and what does their celebration show Scrooge in Dickens' A Christmas Carol?

The Cratchit family is grateful for their feast even though it is meager, and Scrooge realizes that you do not need much to be happy as long as you have people you love.
The Cratchit family reminds Scrooge what it means to be deliberately happy.  The Cratchits are happy because they want to be.  The enjoy each other’s company.  They make the most of small luxuries.  They love each other, and because they do not have much they savor what they have.
Scrooge, who is a stingy miser who spends his nights eating alone and usually just has gruel because it is cheap, is astonished when he sees how excited the Cratchits are about their Christmas feast.  They are making much of little.

And now two smaller Cratchits, boy and girl, came tearing in, screaming that outside the baker's they had smelt the goose, and known it for their own; and basking in luxurious thoughts of sage and onion … (Ch. 3)

The Cratchits all enjoy their goose, and their gravy, apple-sauce and potatoes.  The goose was cooked at the baker’s because they didn’t have a way to cook it.  They were too poor.  When they were praising the goose, the fact that it was cheap was one of the things they were most proud of.  The pudding was also a source of admiration for all.

Everybody had something to say about it, but nobody said or thought it was at all a small pudding for a large family. It would have been flat heresy to do so. Any Cratchit would have blushed to hint at such a thing. (Ch. 3)

The Cratchits would never complain that there wasn’t enough to eat or the dinner was not fine enough. When Scrooge is toasted as the “Founder of the Feast,” Mrs. Cratchit objects at first, calling him “odious.”   Bob tells her to think of the children and she agrees to toast.
The celebration the Cratchits have tells Scrooge that family is more important than money, and you should savor what you do have.  Holidays are about more than spending and presents.  Holidays are about being with the ones you love and enjoying time with them.