Your kids are old enough to understand what's going on, so in general you should be fine...so long as you prep them by explaining to them what they're supposed to do and what's going to happen, and physically walking them through it as many times as possible. Unfortunately, in most weddings at most they'll get to run through it twice at the rehearsal. But if you have access to the facility (e.g. it's your home church), then by all means have them practice.
I found YouTube to be incredibly helpful... just type in "flowergirl" or "ringbearer" and you'll find tons of videos. I had my 3 year old watch a bunch of them over and over and she was able to get it.
Also, bribery can work too. I haven't tried this, but a coworker was telling me that she took her daughter to the store, picked out the special toy together the day before the wedding, and left the store with the promise that if she walked down the aisle and stood quietly through the whole ceremony only then would they go back to the store and get the toy.
Finally, work with the bridal couple on managing expectations. You know what your kids are capable of, so make sure their job descriptions will match up with their capabilities.
For example, f at all possible, don't make the kids stand up the whole time with the rest of the bridal party. That's a recipe for disaster. My husband and I have worked out the "release and catch" strategy. One of us stays in the back to release the flowergirl, the other sits in the first row with access to the aisle to "catch" the flowergirl to make sure she makes it down the aisle. In the last wedding, I caught her and had her sit with me in the front row. Then she kept asking questions in a loud voice, so I had to hustle her out to the lobby to wait until the end to hustle her back to make it in time for the recessional.
If you don't think your daughter will be able to handle it, have her brother escort her down the aisle.
If she can't handle the rose petal thing, florists make this cute ball of roses hanging from a ribbon that she can carry. No strewing of petals involved.
If your daughter is going to wear a hat or some other headpiece, make sure you practice with the item. My daughter kept ripping off the beautiful rose headpiece because she wasn't used to the feel of it.
Good luck!
