I found it easier when all of us brushed our teeth together. Kids like to imitate adults, and it plants the seeds for good hygiene when they see mom & dad brush and floss every night. If bedtime is too late, and she's too tired and cranky, it's fine to brush right after dinner, as long as she's not snacking again before bedtime.
You can try counting the strokes on each side-five seconds on each side, inside and out, left and right, front ones in and out, top and bottom, and there's a minute of brushing which should be enough to clean every surface. If you can find an egg timer so she can watch the sand run through or if you can find a song to play on a tape player, it may help keep her interest. Keep in mind that children don't have the dexterity to do a thorough job until they are about 10, but I can't imagine any child allowing a parent to help at that age. Good Luck!