I think the main risks are from touching farm animals - which is less of a problem for a non-mobile little baby who's in a pushchair. Take some wipes and some antibac gel and most importantly washing hands after touching animals/bars of pens and certainly before eating. Personally i wouldn't apply the 5 sec rule for teething toys/breadsticks that hit the floor at a farm.
Maybe google lambing though - as is it lambing season? There's definitely a risk for pregnant ladies then - I'm not 100% sure about babies (think not, but might be worth double checking).