The kids win, which makes this a tragedy. (Unless you're the kids. Or the lions.) That's what's clear about the ending: 1) George and Lydia get lured into the nursery; 2) the lions get them; 3) the kids win.
What's unclear here is what that "lions get them" means. If you like your horror bloody, you'll say "the lions eat the parents." That's what we say, even though it doesn't entirely make sense. How do fake lions eat real people?
Bradbury carefully sidesteps this question by hinting throughout the story that the lions can affect real, tangible things. For instance, after George and Lydia escape from the nursery at the beginning of the story, Lydia asks if the lions can get out and the door trembles "as if something had jumped against it from the other side" (61). That's a big hint that the lions can have a physical impact on the world outside their virtual one. And we have our suspicions that the lions can affect the real world confirmed beyond a doubt when George says that the lions can't, since George is almost never right.
But because Bradbury doesn't come right out and say what happens (see "Style" for more on that), the ending is open to some interpretation. In one radio version from 1955, Dr. McClean reports that the parents haven't really been eaten, but now the whole family needs therapy. As in, they were attacked by fake lions and now they need to talk it out.
But whether the lions become real and do physical damage or the lions remain virtual and inflict only emotional damage, the story tells us that emotions are Serious Business in family life. After all, the lions are only there to hurt the parents because the kids are feeling so… hurtful.
Whatever happens to the parents, the gist is clear: the kids win. And when we see them at the end, they are enjoying a picnic at the same time as the lions are enjoying their meal. Shudder.