What is the ratio of molassas and pineapple juice for a non-alcoholic rum substitute?
The common "flavor profiles" in rum is molassess, pineapple, and almond. You will commonly find sites offering "molassess thinned down with pineapple juice" as a substitution tip. It's not a coincidence that all these sites don't offer exact ratios. 

Rum when used in baking is primarily for flavoring purposes. Substitutions will therefor not be very strict. Take some molassess and thin it down a little bit with pineapple juice. Do a tasting with each new addition of pineapple juice until you personally think it tastes the "best."