Yeah, if you have to get it on one ticket, it looks like it's going to be expensive -- here's $4500 in J: https://www.google.com/flights/#flt=...3*2.USD.452481

Separate tickets would probably be the way to go... yes, they'll have to recheck their bags (if they're checking bags) but with 18 HNL-LIH flights a day each way, I wouldn't be too worried about misconnecting in the event of a delay, etc. Still looking at $3k each ($2770 for the DFW-HNL nonstops on your dates) but there's no reason (IMO) to pay an extra $1500 each just to get the flights on a single ticket. https://www.google.com/flights/#flt=...2*2.USD.277062