The phrase that could be added to the beginning of the second sentence to create cohesion between the ideas in these sentences is "One reason is that."
So, the revised sentences would read: "A beagle makes the best pet for a family with children. One reason is that beagles have lots of energy for playing."
This phrase clearly links the idea of beagles being good pets to their energetic nature, reinforcing the point made in the first sentence.