I installed one of those door security chains. The chain is screwed into the wall and I installed the receiver part vertically on the door so one would have to slide the chain part up to enter. From the outside, I just slide open the slider on the screen door, reach in and disengage the chain.
Had my 18 month old fall out of the trailer. Luckly she wasn't hurt.
http://www.bing.com/images/search?q=door+security+chain&qpvt=door+security+chain&FORM=IGRE#view=detail&id=653C1B1541421436B6C06562391657A47049BBFD&selectedIndex=53
