Angie already gave such wise advice!

I too agree that LHTH would be super easy to add. I did CTC and Beyond last year (finishing with the start of Bigger half-speed), and LHTH half-speed. Loved it.

We were going to wait on doing LHTH until my ds was 4 yo, but I couldn't wait. He really wanted time with me, and all of those good intentions I had of reading him this or doing that never happened.

So, we just started LHTH half-speed, and he was thrilled!

I agree that "The New Bible in Pictures for Little Eyes" and "Big Thoughts for Little People" would be good choices for both your dc (along with "The Singing Bible). I like the idea of pairs for your kiddos too, and you'd be set up for a very nice teaching rotation by doing it this way.
In Christ,
Julie