The level of tickets don't matter when it comes to boarding. Buy the ticket that is most affordable.
When it comes to boarding, the family board is supposed to be for kids ages 6 and under, but the gate staff do not enforce this unless it is a flight to Orlando in a busy season (summer or holidays) when more than half of the flight are families (my mother lives in Orlando and we fly there regularly).
You get assigned boarding passes based on when you check in. They assign boarding passes in groups of 60, A1-A60, then B1-B60, then C1-C60. The will call the boarding groups up with A1-A30, then A31-A60, then B1-B30 and so on. Nowadays, the As are reserved for people who pay for boarding upgrades, so even if you sign in exactly 24 hours before your flight, you will likely get a B1-B30 checkin assignment. You are allowed to check in 24 hours before flight. Be aware that in the first 2 minutes of the boarding window, likely all or most of the B group will be gone. I've signed in 2 minutes after the window opens and gotten C group several times.
When I have two different ticket groups, I do my check-in on a computer and make sure I get set up about 5 minutes before the window opens. I open two browser windows, navigate both to the Southwest check-in and put in the flight confirmation number and my name in both windows. Then I wait. At the time, I open one, click check-in, switch to the other window and click check-in. I almost always get consecutive boarding passes. I once had one boarding pass slip in between the two groups and when we were at the gate, I just let that person go in front of our family. They were fine with moving up 2 spaces.
Alternatively, if it is a busy flight without too many family boarders, you can move to the family boarding group. That group (separate line) will board between the A and B boarding groups. Like I said, it's supposed to be for families with younger kids, but unless the flight has a ton of children, the gate staff will not stop you from boarding. I've actually had gate staff wave me and my family over to join family boarding when there were not a lot of kids traveling. Like the other PP said, they realize that families want/need to sit together and it saves time if they pick seats together in the first place, rather than having flight attendants have to move people around to get families together.
Last, if you don't want to risk either of the above (both are not guaranteed), pay for the early boarding for both tickets. I averages about $25 per person per leg of a flight (so if you have one transfer, then you'll pay $50 per person). More popular flights, the early boarding is more expensive. Less popular flights, early boarding is less expensive. You should be able to add early boarding to your tickets from the Southwest web-site. You should be able to do it with your Southwest confirmation number. If you can't, Southwest call centers are not that busy after 11pm. Wait until 11pm, call in and you won't wait long to get a customer service rep who can help you add the early boarding to both tickets.
I have twin 12 year olds and I understand. One would be fine boarding separately and finding us. The other is more shy and would be very anxious boarding separately. You know your child.