To train boys and girls in their own special way:
Try using a potty chair -- her feet will touch the floor, which relaxes her pelvic muscles. If she's using an adult toilet, give her a step stool.
Limit spray by having her sit all the way back so her bottom and vagina are over the potty opening. Encourage her to sit with her knees apart, which will also help relax her pelvic muscles.
Teach her to wipe (or pat) from front to back by letting her watch you. Keep her entertained and sitting with books, stickers, or music next to the potty.
Let him use a potty chair to pee if he's not ready to stand and aim. For pooping, use a potty chair or toilet-seat insert (plus a step stool).
Have him push his penis straight down before he sits on the potty chair to avoid scraping it on the splash guard. If he's standing, be sure to position him, feet slightly apart, directly in front of the potty.
Have him watch his dad, or show him how to aim his pee into the bowl. To improve his aim, drop a few Cheerios or goldfish crackers in the toilet, then offer him a fun sticker for hitting a few. If he's making poop, give him a book or play some music to encourage him to sit.