All bakfiets models come with (optional) rain tents, so young can kids stay dry and out of the wind. Cold is usually more of an issue than rain on longer trips. In a bit of snow, I feel a lot safer on my bike than in a car. Just adjust speed and do controlled braking to feel the grip (or lack thereof). It helps a lot that I live in a flat country (Netherlands), as snow/ice and slopes are a really bad combination with cycling.