I know this topic has been going on for YEARS but cannot seem to find the consensus on the latest!

We are about to book flights on LH via United using United miles. It is from NAP-FRA, FRA-TPA. (All LH flights). It is a Economy X code (have enough miles for Business but not available!)

We have miles on other airlines but deferred to this due to the fact the config was 2-4-2 and figured we could make it work in the 2 seats as we liked the timetable the best. We are going with our 8 & 10 yr olds so want to sit next to each other on the 2 side. However, if we cannot select in advance we may reconsider the other airlines (where we can actually fly Business, albeit 2 connections and a longer day).

Does anyone know currently if you book a X fare via United portal but on all LH flights if we can pre select seats for those flights or do we have to wait until 24 hours before?

ANY guidance appreciated!! Thanks in advance!