Welcome to the LANPASS forum pdxasflyer
First time on LAN?
Which LAN are you flying? Are you saying you cannot get the chance to purchase a seat or you cannot select a seat for free? I didn't follow that.
I cannot remember a time when I couldn't choose a seat, even when I was without status but that was before they sold seats too.
Really you need to say which LAN you are on and your route. For example Peru domestic I don't think you can even fly base unless you are Peruvian.
I also don't want to sit next to your children so the wise thing to do would be to get to check in nice and early and grab seats together, if you cannot choose a seat in base then probably 1/3 of the airplane can also not choose a seat.