Childcare is a huge concern for every working parent, being both frequently expensive and often hard to find in the quantity and quality that they require. From this perspective, having somebody in the extended family who is both available and willing to take care of a kid as and when it is needed is nothing short of a blessing. It is a luxury that not all moms and dads have.
However, for those parents who do find themselves benefiting from this, it is important to show some basic respect for the individual who is doing the babysitting. This includes not constantly expecting them to drop everything at the last minute to do childcare, and finding mutually acceptable solutions if a kid is being difficult. Unfortunately, many who get family babysitting privileges take it for granted, and then start to get annoyed when things end up not working out for whatever reason.