It's not that weird if he got married at 15, since he just said that Halflings reach adulthood at 14 yo. So technically he got married at his early 20s in human age which is pretty normal. The biggest question however is the pregnancy period for Halflings, due to their short lifespan, since he has 3 kids (so that's 1-3 pregnancies).
If Halflings have a lifespan that's about half of a human, we can assume that the pregnancy should be more or less the same, right? I'm not a biology major so I don't know how it works.