As someone who has tried (sometimes in vain) to teach various friends and my girlfriend to surf I guess I could share a couple of tips.
1. The first thing I would try is getting the biggest foam board you can (9ft?) and then, on a small day (1ft or less) just tummy-ride a few waves with her lying in front of you, so she get's used to the feeling of moving across the water quickly, lying on the board and with you behind her for security. You could even kneel up if you're stable enough to give her more room on the board.
2. When she's ready to have a go on her own (maybe next session) choose a nice big foam board you know she'll float on. I don't know much about kids that age, having none myself, but I reckon she won't be strong enough to move something which is longer than 7-8ft (and won't need the extra floatiness of it to get going). She won't be paddling much as it'll probably be too wide, so you'll have to stand next to her and push the board onto a wave.
3. You gotta let her take control and try not to give too much advice - there is a lot to learn and most of it is trial and error, even when you know what you're supposed to be doing. I think the hardest thing is the balance between letting someone make mistakes and avoiding them getting injured. If she has a frightening time at any point in the early days, she'll be really resistant to keeping on trying. So make sure you go on the smallest possible day, and in the warmest water you can, and (if remotely possible) on an uncrowded beach.
Don't forget sunscreen, and perhaps your partner armed with a camera to record her first ride!