I'd honestly say this in a soft/normal voice level and tone:
"Excuse, ma'am. I'll let your children sit here as I do understand their magical experience is an important part of your vacation, so long as they remain seated and don't cause too much trouble, but please can I kindly suggest that the next time just ask before just assuming, please. Our family has been sitting in this spot for about 2 hours. I understand how hard it is to find a spot 5 minutes before, so again it isn't a problem this time, but please just take that into consideration in the future. Thank you."
And if I was a cast member and a small argument were to arise between two families in this situation, I would probably say pretty much the exact same thing. You really do have to pick your battles and being willing to compromise. It's not worth making the situation worse for a guest that is ignorant, rude, and unprepared, but it also shouldn't be acceptable for those guests to feel that it is ok to do it. Raising awareness of how other parties may perceive actions and how to think about how their actions may affect others is the only way that you are going to suggest positive change. People will respond negatively if you create conflict for them, but if you are genuinely interested in working with them to create the best situation for the both of you (if with limited time, attention span, and other distractions), they are more apt to remember that and even possibly return the favor to another family in a similar situation at a later date.
At the same time, if you see children having a hard time seeing and you can afford to make a compromise, ask the parents (and children) if they would like to sit closer to see better. The parents that don't shove their kids in front will be thrilled and therefore rewarded for just accepting the view they chose, but now you made their night and again, they may be more apt to remember that and possibly return the favor to another family in a similar situation at a later date.