I'd agree, but I don't think everything else will always be equal. You're unlikely to get the same square footage for the same price. There's a reason that most bigger houses are two stories. You have less exterior and roof to maintain, your house takes up less of your yard, and it's usually cheaper to build. On the other hand, your roof and walls are more accessible so it goes both ways.
If your main concern is kids falling down the stairs, I don't think that concern is really justified. First, not to be callous, but kids are pretty durable and rarely get seriously hurt falling down the stairs. Second, kids are pretty smart and figure out how not to fall down the stairs. I have a 3 year old and we have stairs and he has literally never fallen down the stairs. When he first started crawling it took him a bit to figure out how to turn around and go down backwards, but he worked it out. Third, if you really are concerned, get a gate for the stairs. It's a pretty cheap fix.
If you have other concerns with the layout, then it becomes more personal. As was mentioned, stairs are more of a problem for old people than kids, mostly because old people are harder and more awkward to carry. If you're worried about sleeping on different levels from the kids, can you just put all the bedrooms upstairs? Basically, there are lots of rubs, but I would lean towards a two story house.