A judge could choose to excuse you for any reason that they find compelling enough. However, watching children that are not your own will probably be seen as a hardship for their parents, and not for you. Therefore, I would guess that most judges will likely not excuse you for that reason.
However, the judge's job is to take all the facts of your situation into consideration. So if there are further factors that make you situation an extreme hardship, make sure you speak up and let the judge know when you are given the opportunity.
You may have to raise your hand when they ask for any hardships the jury service may cause on any of the jurors, and you may have to speak in front of everyone. Just remember that they hear these explanations by potential jurors all day long and your excuse or reason won't seem petty to the court. You will be taken seriously, and even if the judge denies your request to be excused, it will be done respectfully. Remember, speak up!