It really depends on their age and what they're into. Our RB is 7 and the FG is 9. I got the FG a really cute doodle book with a set of markers and put them in a cute little "purse" for her. FH got the RB some legos.
The flower girl and ring bearer are my little cousins, ages 4 and 5. We are going to be making a little goodie bags for them filled with things they can do at the ceremony and reception. I'll probably have a coloring book and crayons, little toys, disposable camera, and a stuffed animal or doll of some sort. I will probably make a personalized T-shirt for both of them as well to change into at the reception.
We have 3 FGs and 2 RBs. For our FGs we got them sweaters to wear with their dresses, and I made them each a pearl necklace. We bought our RBs silver piggy banks from Kohls and came in under budget (only $13.50 each with a coupon) so I'm making up the difference in quarters for them to put in their banks.
I am making my gifts to them... Were big crafters in our family so a lot of my wedding had be diy... Im making my flower girl who is my niece and god daughter a bracelet with pink swarovski crystal beads, star shaped pearl beads, with a silver star shaped beads with a clear swarovski crystals in them. I also got her a little clam shaped pearlized trinket box to go along with the bracelet...
My nephew is going to be the "flower girl escort" we didnt want him to feel left out so we gave him a job too... Im buying the letters of his name made of wood and the base to set them on and Im painting them in different types of camoflauge since my bro (their daddy) is in the Army! Whooah!
Im still undecided on what to do for my son who is our ring bearer... I did get him a shirt that says "official ring bearer" and on the back "ring security" with a little badge to go with it...
One of my favorite ideas was to give them little "bribes" at the ceremony. I gave the parents little canvas bags with "goodies" inside. You can paint their names on it, or get some cute iron on patches. I filled it with little toys from the dollar store: coloring book, crayons, small doll, cars, stickers, sun glasses, Clean Snacks (gummies, gold fish) etc. It helps get the kids down the aisle, and gives them something to amuse themselves with during the ceremony - and stuff for the reception if they are invited to dinner.