The AS site is pretty explicit that AL members are given access to partner lounges (including AC's) + Spouse + children under 21.

The AA site talks about access for various categories of individuals but doesn't mention AL member rules accessing AC's (it does for AC members accessing AL's where it says under 18).

If I were a betting man, I would bet that if the OP did this they would probably get turned away at the AC. At that point the OP could buy a day pass for the remaining child then complain to AS that their stated policies aren't being followed and they should refund the cost of the day pass. Might work, might not.

Also it is worth noting that in October, AS is revamping their membership structure into two categories. While existing members get grandfathered in for the remainder of their membership year, at that point it may be worth it to avoid this situation and get a spouse membership at the AC or a Citi Executive card with the spouse as an AU.

AL Locations only: $450 (or $350 for AS elites)
AL Locations plus partner lounges: $600 (or $500 for AS elites)