The basic requirement for content filtering and time control is, traffic must passthrough the pcWRT router.
If your Internet speed is 50Mbps or lower, I’d suggest that you connect it this way:
Internet
|- pcWRT
|- Linksys 1900AC
|- Other 1900AC AP
I.e.,
1. Connect pcWRT to your modem, turn off wifi.
2. Connect Linksys by wire to the pcWRT, configure the Linksys as AP, turn on wifi.
3. Configure the other 1900AC router as AP, connect via wifi to the Linksys.
DHCP is served by the pcWRT. All Internet traffic in home can be filtered by pcWRT.
If your Internet speed is 100Mbps or higher, pcWRT’s Ethernet port is not fast enough to get the bandwidth you paid for. I’d suggest that you connect the Linksys to the Internet as a router, configure the other 1900AC router as AP, and connect via wifi to the Linksys. Attach the pcWRT to either the Linksys or the 1900AC AP.
You’ll have two networks this way, one from the Linksys, another from pcWRT. Both serving DHCP. Have kids connect to the pcWRT wifi. The Linksys network will have no filtering. Another downside is the pcWRT wifi will have no repeater to extend the range.
A third variation is to attach pcWRT to the Linksys, then have the other 1900AC serve as AP off of the pcWRT wifi (only 2.4Ghz since the pcWRT is single band). I don’t know if you can split the 1900AC to extend 2.4Ghz from pcWRT, 5.0Ghz from the Linksys. There are also two networks in this variation. The Linksys network is not filtered.
If your Internet speed is between 50Mbps and 100Mbps, you can perhaps try different ways to see which works best. Thanks!