I'm late to the party! We had our nieces and nephew (ages 1, 3, 5) for 3 days. We love them to death and had a blast, but man, are they are lot of work! I don't know how you parents do it!
Friday: swimming with two nieces (note, this wasn't really a workout)
Saturday: rest day
Sunday: 12 mile run while DH stayed with the kids during their nap time. Run was awful, legs felt heavy and I just couldn't get any faster than 10:15 mile and miles toward the end were 11:00+