Ah, ok, if they are 4 and 2 I think you can easily skip the trailer. Unless you plan more kids
They will be enjoying the ride way more with the seats. Plus very soon you can switch to a trail-a-bike (not runabout, mixed up the name) anyway (i would do so now if i were you). The 4yr old can probably pedal on her own? Then you don't really need the xtracycle and can use whatever seat you like.
+1 for an xtracycle and well planned itinerary.