Even if you book assigned seats together, there's no guarantee that there will not be an equipment change and you could be reassigned seats apart. Most people are good about accomodating young children. I've moved plenty of times (when they asked nicely). So you to decide if you want to take that risk or find another flight where there are open seats together.
First, do NOT count on the bolded. Yes, you can try, but don't COUNT on it.
Second, even at the gate, Delta will TRY to put you all together (more in a moment)
Third, as mentioned, even if you are assigned seats together, that doesn't mean they'll stay together.
Fourth, I'd start "prepping" the 7yo they could be sitting by themselves.
Now, my stories...
1) A couple years ago when flying to WDW, we were sitting next to a family that had gotten separated. Instead of the three seats together they had originally booked, there were two in one row and one in the row behind. One of Delta's "changes" had forced them into different seats from what they had booked.
2) When heading on vacation this year, I had all our seats setup. 2, 1, 2. All three kids get window seats, teenager by her self, the two younger ones with me & DH. The first leg of our trip left 90 minutes late because of mechanical issues. Needless to say, we missed our connection. Delta put us on the next flight to our destination. But they didn't assign seats. When we got to the gate, I told the gate agent that of our 5 tickets, two were younger children (6 & 8). After about 20 minutes, they got us four seats together, and 1 two rows up. I simply wanted two seats together for the 6yo. The rest of us could deal.
3) On the way back from that vacation, at some point, Delta changed the 2, 1, 2 arrangement I had, and sat us all across in one row. I didn't realize it until too late, and it wasn't worth raising a stink.
So yes, you COULD get seats apart from each other, but probably not. Unless SW is available to you, you might as well stay with Delta.