Best way (and cheapest) from St Claire Sur Elle (Manche) in France to Zooparc de Beauval in france?

There is no direct train line between these two places, in fact, you'd need to go to Paris first and then take the train to a station near the Zooparc. So you're actually looking at a full day's travel each way! As you've pointed out, it a 4 hour trip by car, which would be much faster than the train, but it's still a long way to go. I think it would be better to give the Zooparc a miss on this trip, the children are going to be bored and exhausted by the trip and will probably not be in the mood to appreciate what you are trying to show them.