We just took DD (22 months) there last weekend (rainy day out). She had a blast and didn't want to leave. We went late on Sunday and had the place to ourselves. There are 2 a few hands on areas (general store, kitchen and chicken coop/cow) that DS loved. Its worth a shot, I'd go on a day that the weather isn't too great. There wasn't anything outside this time of year, but may be different in Spring.
Re: Loudon Heritage Farm Museum