No, you cannot "make" someone visit with their children, but you can incentivize, motivate and encourage. One motivation technique or incentive we (as divorce lawyers) often employ is to craft the agreement, or court order such that if a visiting parent misses their scheduled visit, they pay the costs for the custodial parent to have a babysitter for that time period. Of course that is a negative incentive, so we prefer positive ones like perhaps offering to be flexible with the times or to do the work for them (like to plan a birthday or holiday party and let the other parent come enjoy it without having to prepare, contribute or clean up. No, it is not fair. But it may give the children a chance to see the other parent.
It is a difficult issue, because often the custodial parent is simply trying to have the other parent visit, because it is good for the kids. A therapist may help and parents, family members and friends of the visiting parent if approved very politely (and perhaps under the guidance of a psychologist or other expert) maybe be able to help facilitate the request. But the hardest part is always explaining to the kids why the other parent doesn't seem to want to visit. Kids are smart and they figure it out, so my advice is often to simply reassure the children that the other parent loves them and has to figure out his or her own way to express that love, but that eventually he or she will.
Sooner or later the children will be adults and will develop their own connection with (or isolation from) the other parent. How you handle it until then is what the kids will know and hopefully love about you. If that is your burden, do your best to put a positive spin on it. You get to see your kids much more than most separated parents, and you get to have the most input into who they become. It is actually an opportunity many separated parents wish they had. Yes it is hard work, but thank goodness your kids have you, one good parent, and that is more than many less fortunate children in this world have.