Children are tiring, bring in the pillows
These young kids are energy sappers, let's come out and be honest about it. Many new parents just get totally exhausted playing with their 1 month old baby for 10 mins - now wait another 5 years until that baby has grown a pair of unstoppable legs and an unclosable mouth and see what happens, you can do the math. Actually that's not true, in theory, the parent should have acquired extra battery-packs at that point which would allow him or her to last another couple more hours. That's the miracle of parenthood - extra energy when you need it most.
If not, just call your best friend over and he'll entertain them for a couple of hours.