This is generally true, and offering row 4 for row 3 should help unless 4D prefers an aisle seat as you mention. If you can't get it done by having the passenger paged at the gate, I'd board first if I could (which should be doable if you're at the gate early enough to try the paging idea) and seat your son in 4F and your daughter in 4C (assuming he's the 6 y.o. and she's 4). If I were in 4D and boarded to find someone's 6-year-old son next to me and his 4-year-old sister immediately across the aisle, I'd likely accept an offer to trade to any other seat in F, and I certainly wouldn't cause much of a delay by lingering over the decision. If he/she declines, 3C may overhear and offer to switch, or may be more likely to agree to a request to change seats if he/she thinks the alternative is to sit directly in front of a 4-year-old. If nothing works and you prefer the aisle to keep closer tabs on your son, you can always switch with your daughter at any time.