Inviting kids to the wedding is a personal choice for the bride and groom, certainly. If they are included, there will need to be some plans for keeping them comfortable and entertained; wedding days are far too long for their attention spans.
Having a kids' room at the wedding may sound like a great way to keep them happy so adults can enjoy the reception. However, don't think that this comes cheap: rates on the east coast range from $95 to $205 per child for a 5-hour reception. While this comes with a full tea party, a character, and the option for a candy bar (yay for sugaring them up and then sending them home!), this is more than a lot of brides spend per adult guest.
Keep in mind, if your guests "expect" babysitting, then they aren't the ones who would have made the efforts to attend your event, anyway. But, if a kids' room is something you feel you need to keep the majority of your guests happy, then plan carefully.
If you want to set up your own kids room and hire sitters, check with your local chapter of the American Red Cross (who offers baby sitter certifications as well as First Aid and CPR certifications) or your local Nanny Service for guidelines on babysitting groups of kids. They can help you figure a good baby-to-sitter or kid-to-sitter ratio. Remember, the more young babies, the more sitters you'll need. Be sure that anyone you hire has impeccable references (or works for a reputable agency), is First Aid and CPR certified, and is experienced with children in the ranges you'll be inviting. A Nanny Service might be able to refer some of their weekday nannies for your weekend event. And, the local university's "future teachers" organization may have some members with enough experience with the age kids you're inviting.
A kids room should also be equipped with entertainment geared for the ages attending: movies, games, puzzles, and toys. Babies will also need cribs or "pac-n-play" sets for napping differently than they do in a car seat.
Any coloring activity should be "clean". Think crayons and sticker books instead of markers or paint. Friendship bracelets are fun for girls (just make sure beads aren't too tiny for the youngest kids). If you a game console and tv for the room, be sure the games are a good fit for both genders and all the ages you're inviting.
No pieces of any toy in the kids room should be too small for the youngest child in the room. The last thing you'd want is for a baby to eat a doll accessory and choke during your reception.
A kids room needs to be equipped with an emergency kit that the sitters can access. Be sure to include plenty of "fun" bandaids because when one kid gets one, they'll all want one.
If you are hiring any entertainment for the kids room, be sure it's age appropriate for the youngest child. Don't hire a clown to make balloon animals for the elementary-age kids if you have toddlers in the same room. Instead, consider a magician or a storybook character.
In lieu of a candy bar, consider heathy snacks and goody bags for them to take home.
And don't forget to hold the bubbles to the end. They'll love rejoining their parents for your sendoff and getaway!