I would suggest tung oil, since it dries and hardens naturally. Mineral oil is non-drying and evaporates over time (which is why, if you use it for butcher blocks or the like, you have to redo it periodically). There have been a number of "scare articles" about ingesting mineral oil claiming it is carcinogenic, but I think most of those have been debunked (well, not really debunked, but clarified that low grade mineral oils may be carcinogenic but that the refined mineral oils in "food grade" mineral oils are probably ok). Mineral oil might be good if your little one is suffering from constipation, though, as it was traditionally used as a laxative - I think this is no longer a common practice.