Florence,
I'm wondering if you could consider doing both LHFHG and Bigger Hearts at half-speed?
For example you could do the left side of Bigger Hearts on the day your younger one was doing the right side of LHFHG. That way your oldest wouldn't really want/need to sit-in on the right side of LHFGH as it is more about the 3R's, but your 5 year old would have the needed 3 R's practice alone wtih you. Then, on the next day, you could do the left side of the LHFHG and have the 9 year old child sit-in for those lessons and then do the right side of Bigger Hearts for the older child too on that day (as that is the 3R's and science).
In that way, your oldest could reap the benefits of sitting in on LHFHG's left side and of still moving forward in Bigger Hearts, and your little 5 year old could keep moving steadily forward too, but your day would be MUCH shorter.
I know it might confuse the order of history events a bit to pop back and forth between LHFHG and Bigger's readings, however I think kiddos already do that often with what they're reading on their own.
If this worked for you, you could keep that plan for two years then, or could reassess at the beginning of next year to see if you want one or both to go full-speed. You'd likely enjoy your time with them more if you weren't as rushed for time too.
Just a thought!
Blessings,
Carrie