The mishna says that for any worker who is hired by the hour, one only has the remainder of that time period to pay. If one would hire a babysitter for a few hours at night, the money must be payed before morning, otherwise the employer would be in violation of לא תלין פעולת שכיר אתך עד בקר. It is true that if one doesn't have money, he is not in violation of this issur, but it is hard to justify that a 5 minute trip to the closest ATM machine is considered not having money [even if the employer has the option of collecting a debt that is due, the shulchan aruch considers as if he has money, and would be in violation for not paying his worker. The chofetz chaim (ahavas chesed 1:9: footnote 21) is mesupak even if one is capable of borrowing he may be obligated to do so in order to pay his worker, but even if he isn't obligated he should borrow to be mechayev himself and enable himelf to fulfill the mitzvas aseh of b'yomo ti'tein s'charo]. Therefore, it is important to realize that this can be a d'oraysa violation.
Even if the babysitter isn't tovei'ah the money - often young girls are embarrassed to ask for their payment, there would still be a violation. Although the gemara says that the issur only applies if the worker asks for payment, the chofetz chaim (ahavas chesed 1:9:footnote 29) explains that since the limud is from אתך - לדעתך ולא מדעתו, meaning by the worker not making a claim he is indicating that he agrees to the delay. This only applies to a case where the worker is not claiming because he is agreeing to a delay, but if the worker is just embarrassed to ask, it is not considered to be m'da'ato and therefore the employer is in violation of this issur. Furthermore, the chofetz chaim cites the sha'ar mishpat that when the worker doesn't claim the employer avoids the la'v but is still in violation of an issur.
Even if the babysitter is less than Bar/Bat mitzvah, the chofetz chayim (ahavas chesed 1:9:footnote 16) proves that there is an issur even by a child. From the fact that the Rambam writes that a worker who is a child can swear to collect his payment during the time that the should be payed, which implies that after the time passes, the employer has the upper hand, as the gemara 113a explains. The Rambam implies that the employer has the upper hand even by a child. Since this is based on the chazaka that the ba'al habayis would not be in violation of בל תלין, we see that there is a violation of ba'al talin even to withhold the payment of a child. The chofetz chaim points out that one may not even delay the payment of a child worker, and certainly if he would promise to pay something (even as little as a candy) and not follow through, it would be a d'oraysa violation. I am not sure if this same prohibition would apply to one's own child. If one would tell their son or daughter that they will pay them for a chore by taking them for ice cream, would they be obligated to keep their commitment that same day? In a case where it is understood that they won't be going for ice cream that day (such as when they had fleishigs for dinner), it is similar to the gemara 111a where the expectation of waiting for the market day, will enable the employer to avoid the issur, so too here the parent would avoid the issur. But what if it is possible to go for ice cream that day just that the parent is too tired (happens to all of us!) - is there an issur d'oraysa?