Daybeds are much higher off the ground than a twin is normally. For the trundle I would think that the day bed would be much easier to manipulate.
Have you considered a different option for the room? I had some beds similar to this when I was younger and they were very good for 2 kids sharing a room because they took up only wall space and not space out of the middle of the room. The floor did not have to be perfectly clean every night.
Ours were much simpler that this set but very much the same idea. I loved them because the floor space being open all the time just made the small room look and feel much bigger. You could put 2 twin beds on Hollywood frames and put one bed against both walls while the other is just against the one. Like an L. Just pick up the shelving unit and leave it off....
http://www.thefurniture.com/store/ASHL-SD-MSTL-TWIN-BDRM-...
Here's another set, less girly.
http://www.pbteen.com/products/store-it-bed-and-corner-un...
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I applaud your decision to not get bunk beds. My bf in Jr. High and High school had a son who died after falling off the top bunk. He was in elementary school. I just cannot stand the thought of kids having them.