I think it's a great idea! I'd write in with the invite (even just copies) a little explaination about the donation. I would not think it odd at all, especially after reading about the no present birthdays on here.

I would be a little surprised and find it unusual, but interesting none the less.
I think I would be curious if your child wanted this, or if they really wished that they could have some gifts instead. I would just be curious.

I like the idea of the card that has a full explination.
Maybe you could turn the parties whole theme into something that has to do with pets as well.
You could watch Disney's "Homeward Bound" and get your child a puppy as a gift.

im having a bday party for my 3 yr old without gifts. in fact, my friends (her playgroup) doesnt know about it either...they just think they're coming over for playgroup.

we're having cupcakes, balloons, hats, all that but they will all get a little playdoh thing, the blow horn things, a balloon and some stickers.

why? cause my daughter is just enamored with the idea of candles, balloons and her friends. she'll get some presents at her "real" party with our family. the reason why im doing a "surprise" party with her playgroup is that already these parties are too much. a bunch of mothers and fathers (esp. the fathers) who dont know each other and looking pained for being there at a 3 yr old party? and these gifts that were being given are like 20-30 dollar gifts! its too much hype for me.

i just want the party with the fun and without the required run to target/walmart 2 hrs before. im giving my friends a pass!

i love your idea of stuff for the animal shelter! bravo!

If the children are old enough to handle it - maybe the shelter has a brochure or flyer you could copy and put into the invitation so everyone could learn more about the shelter. You might also call them and ask what their needs are - some shelters are sponsored with food, so they might not need it. Some need towels - tons of them - the old/raggedy ones are great. Then you could attach a short "wish list" so that you can bring the animals what they really need.

If it has a lot of graphic information about how many animals are killed each month, etc - I would leave it out. My DS is very sensitive and he would obsess and obsess about something like that. "why do they kill the dogs? why? why? why? why?" etc. I'm not sure I would like the mom that caused me to end up with 15 dogs! What a great kid you have - she already gets it at such a young age - you must be a great mom!