I loved Warm Bodies and I thought the film was great, but they completely left out R's kids! Seriously, why would you do that?!
Anyways, if you've read the book, you know that R's not exactly "Parent of the Year". In fact, he's a bit absent, gets lost in his own thoughts, and has a tendency to just wander off completely. He does his best though.
Edit: As a couple of people pointed out, they did acknowledge the kids but were just left out of the storyline to prevent the film from becoming too confusing. And, you know, like, keep it under seven hours long