The ending makes no sense.
1) After the humans discover the dead rat, they realize that LADY tried to save the baby from it... but why do they assume TRAMP helped too? For all they know, he was a dog who somehow entered the house (probably the same way the rat did). And even if they did put 2 and 2 together, why rush into the car, specially when it's raining and when their baby needs to be calmed? Why not call the pound instead? It's not like they kill dogs the moment they arrive.
2) Why does LADY forgive TRAMP? He saved the baby, but he had already saved her before. The issue wasn't that he was heroic; it was that he was a womanizer.
3) If a stray dog did something to help you, you would be thankful... but would you adopt them? He could have diseases and you have a new baby.