I find it funny that this is coming up on the WN forum. I have found it drastically easier to get seated with my children without paying extra on WN than on any other airline I've flown since becoming a parent. The other airlines charge for so many seats that it has been challenging to find seats together.
While I can imagine a handful of scenarios that would make this hard on WN (late arriving inbound on a connection, etc), my experience is that under normal circumstances WN comes out ahead in this area.