Hiking trip went south, any tips for next time?

Took the boys on a trail we've done before but didn't account for how much slower my 6 year old moves now that he's getting heavier. Turned a 2 hour hike into almost 4 hours and everyone was exhausted and cranky by the end. My legs are still sore. We made it but it wasn't fun. Any veterans here have tips for hiking with young kids? Should I just do shorter routes or is there some trick I'm missing?