Is it an issue for you or the older kids if it is some of the time? I would agree with aeh that not all kids are hostile to younger kids.
I remember playing a lot with a girl 3 years younger (she was amazingly gifted in running... our PE teacher had her doing the 50 yard dash against our fastest *boy* who was 3 years older than her and she tied him in raw speed) - we did a lot of physical play outside that involved running and agility and were matched well (I was more agile but she was faster). And I recall my most likely PG friend (she is beyond what I could keep up with) really liking having my much younger sister "tagging" along especially when we were traveling... so not all kids are as age conscious.
