I know this may not solve everything, but for each of our two kids who moved from lap-child to their own seat, we talked it up about how big it was that she gets her own seat. If flying an airline (not WN), that allows early boarding for families, we'd go down the row and let them find their number and then show them where its at.
I found that works better than just saying, "here's your seat and you're going to have to sit here now." I know each kids different, but that's what's worked for us.