I'm going to go a step harsher. The monster that resides beneath Derry isn't responsible for the fact that none of the Losers have children. It could care less. In fact, I suspect Pennywise would love nothing better than to hurt those that hurt it by devouring their children first before killing them. That isn't an option because something else is intervening in subtle ways. You must remember that God or Gan or whomever/whatever sits on the top floor of the Tower helps those that help themselves. This force for good, the light, doesn't intervene directly. There are rules which it has created and follows. The why of these rules is beyond us mere mortals but order versus chaos is a thing. In any event, it is this same force which aids (however slightly and within the rules) the Losers as children which also watches over them as adults. It is this force which denies them children. It is this force which puts harsh things in motion for a greater good. I will explain why.
Just as Pennywise knows the reckoning will one day come, and the Losers will return or be called back, so does the force which helped them. Since labels are uncomfortable when we are dealing with cosmic things we don't understand, I shall use the term from the book, i.e. the Other. This force, which is clearly good (if harsh), knows that to fight Pennywise as adults, the Losers must maintain their connection (however distant) with childhood. My argument, and it is open to debate, is that once you have children of your own... it is a rite of passage... a crossing of the Rubicon... the point at which you are (for most good people at least) irrevocably an adult. Good people, those that stand, make their choices by what is right for their children and have to be responsible in ways which are mutually exclusive from the mindset of a child. I'm not making value statements here, merely commenting on how the world changes for parents (at least those who are good people who care about their children). You just don't see the world the same way. I submit that had any of the Losers had children, they most certainly would not have either come back to Derry setting that responsibility aside, or even if they came they would not have been able to reconnect to the children they had been. That bridge would have been burned. Good people still, but not longer able to cross back into the mind of childhood to do battle.
God/Gan/Tower works in mysterious ways. It also works in painful ways. I've approached this subject before, but it clear that the Other was working to get rid of the horrible force that is Pennywise for a long time (within the rules). There needed to be just the right kind of people (the Losers) who stand and be counted. God/Gan/Tower can help them as long as they help themselves. This force for good has to be patient (and it was). There is also a ruthlessness in the approach. Consider Georgie; what/who made sure that he would be in exactly the right spot and right time to encounter Pennywise? Why was Georgie important? Without Georgie, Big Bill wouldn't have had the perfect nudge to become the anchor of the Losers. The dominoes are starting to fall. Something had to push the first one. There was turtle imagery which transfixed Georgie in the basement. It was an odd moment and seemingly unrelated to larger issues, but there nonetheless. Horrible as it sounds, and odd series of events had to create the perfect timing for Big Bill's little brother to be there at the right time and place to be killed. Moreover, some adamant force intervened to keep the Losers childless despite best efforts until their job was done. I submit the force is one and the same. The book posits a series of cosmic ideas which are difficult but also ring true:
1. There is a force greater than ourselves which will help us and moves the world in the right direction.
2. We do have choice as individuals and those choices decide if the world continues or falls.
3. That force which is greater than ourselves will help those who helps themselves, i.e. those that choose the light.
We are talking about that long debated problem of predestination versus free will. The novel (as do many of Sai King's works) puts forward the notion that the two are not mutually exclusive. It puts forward the idea that God/Gan/Tower does decide much of what happens in our lives, but that ultimately the picture is completed by people (at least some people) making choices. From our perspective, the force that is the Other is titanic and monstrous in a way not unlike Pennywise. This force allows little children to be devoured by monsters; it even puts some children in that monster's path. We cannot understand this methodology because we are inside the picture, we do not see the whole picture as this Other must certainly see it. We accept on faith or instinct that the Other does know what it is doing. We understand the harshness, or at least we try to understand it to keep the faith. We are talking about meaning, that order from chaos that I spoke of before. Why do bad things happen to good people? God/Gan/Tower must have a reason.