I sympathise. In my experience, it's taken 3 - 5 days to get over a 7 hour time difference. My kids very good sleepers usually. (Usually they only suffer from jet lag going East ... so going from Asia to UK they are normally fine.)
What works for us to get back on track is.
- Wake them at pretty much normal time and then do your regular "routine". Eg, if my kids normally woke up at 7am, then I'd get them out of bed sometime between 7 and 9. So a bit of flexibility but not much. Then normal meal times and naps as much as poss. (Maybe a bit earlier/later depending on what they needed.) And I'd wake them after a reasonable nap, not let them sleep on.
Lots of outdoor activity. Sunshine/daylight really good for resetting body clock. Plus a bit of running around wears them out!
Normal bedtime routine and in bed at similar time. (Coming back to Asia, I do a later bed time for a few nights, bringing it forward an hour or 2 night by night ... UK I do normal bed time.)
If they wake at night in first few days, mine were often hungry, so would do toast/cereal. I always kept lights low ... so just a sidelight. Watch videos for a couple of hours. After an hour or 2, I'd put them back to bed.
After a few nights, I wouldn't let them out of bedroom - would keep light off entirely if possible ... but might keep them company/lie down with them.
Key is to keep lights low/ quiet TV only ... no rowdy play! And then to start day as normal as much as is humanely possible.
Not sure about the big 12 hour time shift ... hopefully you'll get some more advice soon.
Good luck!