when baby juliet fell over what did the nurses husband say and why do the nurse think this is funny?

The nurse uses crude humor to describe Juliet getting a bump on her head when she was a toddler. 

Yes, madam, yet I cannot choose but laugh
To think it should leave crying and say ‘Ay';
And yet I warrant it had upon it brow
A bump as big as a young cockerel’s stone,
A perilous knock, and it cried bitterly.
‘Yea,’ quoth my husband, ‘fall’st upon thy face?
Thou wilt fall backward when thou comest to age,
Wilt thou not, Jule?’ It stinted, and said ‘Ay.’

A cockerel’s stone implies a penis and testicles. The husband goes on to makes a dirty joke about Juliet reaching puberty.