You're proposing to book two separate tickets, as Southwest does not have interline partners. At a minimum, I would book the later flight, and I would give serious consideration to not doing this at all and booking it all on one ticket.
With two separate tickets, you are responsible for your own connection. You must go through immigration, collect any checked bags, go through customs, change terminals, wait in line at the check-in/bag drop counter, check in your checked bags before the deadline, go through security, and get to your gate. If your inbound international flight is late or you're stuck in line at immigration, Southwest is not responsible for your connection. They may, as an unofficial policy if you notify them ahead of time, put you on standby for the next flight, but otherwise would be in their rights to charge you for new tickets on the spot. And with 3 kids, there may not be room on the next flight for your entire party, so you could be waiting longer.
The terminals (terminal 4 and terminal 1) are not close either. There's a inter-terminal shuttle bus every 10 minutes or so, or it's about a half mile walk. The good news is that at that hour, immigration wait times aren't long, generally not more than 20-30 minutes.
The best approach, especially traveling with kids, would be to book this as a single ticket. This will make the airline responsible for your connection, will avoid the need to carry baggage half way around the airport, and will reduce some of the risk and uncertainty.
If you do proceed with separate tickets, the 9:00 is a nonstarter, particularly if you have checked bags and three kids to get to the other side of the airport. The 11:00 should be doable as long as you aren't significantly delayed anywhere and are ok assuming the risk of separate tickets.