In the first sentence the 'too' doesn't seem to add to or change the meaning.
In the second, using "a boy and a girl" works to suggest they are a pair, in that traditional romantic sense, which the first sentence doesn't, since there they are separately listed.
The 'too' in the second case seems to help that reading. "a boy and a girl, and a dog too" seems to reinforce the separateness of the "a dog" item from the "a boy and a girl" item. Whereas if you'd just put "a boy and a girl, and a dog", one might tend to read it as a wordy kind of list - "a boy and a girl and a dog".
But this kind of nuance may be a little subjective.