Y.,
I love doing kid's birthday parties! I've thrown over 30 of them with 3 kids over the years.
You don't say if you child is a girl or a boy?
For invitations, I always go to the office supply stores, like Office Depot. I look for stationery that's on SALE. Like $1.99 for a pack. Something generic, like dots, or hat/streamers. Even if its just a plain colored border, then I put a sticker or two on them to jazz them up. Whatever's on sale, that's what I get. I have quite a collection now and, since they sell these papers in packs of 25 or 50, I use part of a pack for one kid's bd, then I will use the rest of the pack for one of my other kid's birthdays...usually, I don't use the same invite paper during the same year though. I have a file with all of these papers in my desk. I look for these deals all year long -- not just right before a birthday. Sometimes you can get the coolest looking paper for 99 cents! Summertime is the absolute BEST time of year to look for these sales at the office supply store. Man, I have scored BIGTIME.
Forget the extra expense of envelopes. Just fold your invites into third's and seal with a sticker. I don't know about you, but I have a bunch of weird stickers my kids get from all sorts of places...they seem to end up in funny places, like under the couch cushions or behind their dresser... So, I snag 'em before they can wander off to the black hole and put them in my folder with all the stationery, so I have 'seals' collected over the year and enough for my invitations. You can also make your own by simply cutting out mall photos of stuff in magazine, like pix of POOL TOYS from ads that are out now, or even puppies, flowers, or whatnot and glue-sticking them on to seal the invites. Great project for kids, all that cutting and glue-stickin'.
You can either print out the invite on white, then load the stationery into a copy machine...uuuuh, wherever that might be...ahem, ahem... and print away. Or, just feed the stationery into your printer.
If you do compose the invite on white paper first, you can add things like little pictures your kid has made, their school photo or other pix from the previous year. Glue-stick them on 'floating' around the border. That way, when you slap that sheet down on the photocopier, it will print out your creation on the stationery. I've made invites with my kid's face on a ninja's body, etc., and other cartoon-y stuff. If you have access to a color printer, its even better. If you don't though, I don't thinks its necessarily worth it to spring for color photocopying at the local office supply store. Just stick with black and white. You can always 'colorize' your invites with colored pencils after printing! More opp's for kid participation from your child.
Best,
C.