Wonderland items have actually been available in Happy Pets since March 4th, but the developers waited until today to make their official announcements on their Facebook fan page. Part of the reason is probably because the game is still somewhat glitchy. Just the other day, I noticed my pets running into the wall of the first room, even though there was nothing keeping them there.
As part of the Wonderland theme, you've got the six purchasable in-game store items seen in the above image. Three of them are really special, as they can interact with your pet to change their size: 1) Rabbit Hole, 2) Growth Cake (it's the "Eat Me" cupcake), and 3) Shrink Potion (the purple bottle with "Drink Me" on it)
The Rabbit Hole can be switched on and off like a furniture item, and your pets can jump into it when it's turned on. When they jump back out, the pets would change sizes for a few minutes. Growth Cake makes your pets bigger, while the Shrink Potion covers your pet in a purple swirl and shrinks them. But the problem with the Cake and Potion is that you don't have much control when the pet is affected by them.
These interactive items are clearly not as streamlined and consistently workable like the ones found in PetVille and Pet Society, but it's a fun step in the right direction for the game's makers.
Be warned, all the Wonderland items are scheduled to disappear by March 22nd.