One of our neighbours has a DD a couple of months oder than ours and it's great, the girls spend a lot of time together and enjoy each others company (as much as 15 month olds can)
We regularly look after their DD (and help out with other things but that's not the point) and I have no issue with that and they always offer to return the favour, but whenever we have approached them for help they're always busy, and then when we cancel our plans they're suddenly free again.
my DP doesn't mind and says it's just the way things go, but it's really starting to wind me up, this isn't the only family who are happy to take advantage of our baby sitting facilites, but never return the favour, but they are by far the worst.