My guess is that the first time the pony did it, your daughter let her because she either didn't know what to do, thought it was no big deal, the pony needed to stretch, etc. It doesn't really matter-- she's just a kid and shouldn't be expected to understand horse psychology. But, now that the problem has arisen, you can take the opportunity to teach her a lesson about good horsemanship.
Explain to your daughter that she should do her best not to let the horse drop his head. Show her how low the pony should be allowed to drop his head, and if he lowers it an inch, she should lift his head back up (show her and explain to her how to do this). However, if the pony's been allowed to get away with this for too long, he'll be bound and determined to get his head down-- and your daughter most likely won't have the strength to keep him doing that. So... I would recommend in trying an overcheck-- basically the same as daisy reins-- I just happen to like overchecks better.
Stephanie Bay's advice is quite good-- teach your daughter good positioning for general riding, and good positioning for a troublesome moment (yes, there's a big difference between a proper seat and your "I'm NOT moving!" seat, and most definitely a time and place for each), as well as how to correct her pony when he drops his head.
He's most likely picked that trick up trying to reach some grass, and then realized that if he did it quickly and pulled hard enough, he could either yank the reins out of the rider's hands or simply pull them off balance. Horses in general, but especially ponies, can be lazy, and all but the most well-trained and well-mannered are likely to take advantage of an innocent, unsuspecting passenger. At my barn, we call horses who've been allowed to have their way "kid spoiled--" they've learned to walk all over their handlers, just because their handlers were too small, too young, too un-authoritative (yes... horses like authority-- they like the security of knowing you "know everything"--
), too uneducated, unsuspecting, and unsupervised. We've got a couple horses who are the most excellent, sweet, willing horses-- but you can't sit up there like a tourist or else they slip back into their kid-spoiled ways.
Nip it in the bud now, so your daughter can once again ride without worrying about her pony's behavior and training.