Baa Baa Black sheep
Have you any wool
Yes sir yes sir, three bags full
One for my master, one for may dame
One for the little boy who lives down the lane

There are five characters in this poem – the gentle one who queries and four more. The sheep politely and truthfully answers showing his duty and obligations to the master, love for the beloved and concern for the little boy. This poem portrays politeness, truth, duty, love (for women), innocence and concern for children. Is this not good!
 
Next, let us consider another:

Jack and Jill
Went up the hill
To fetch a pail of water
Jack fell down, broke his crown
And Jill came tumbling after

This poem has great societal and environmental value. Often the water reservoirs are constructed on hill tops for the following reasons: it is easier to supply the water using gravity gradient, it is free of usual pollutions and beyond the reach of people. This practice is prevalent to this day. It appears from the poem that Jack and Jill are a bit naughty. They had gone to fetch water by dipping the pail in the reservoir and perhaps had to rush back and in the process were wounded.
 
And finally the lovely one:

Twinkle twinkle little star
How I wonder what you are
Up above the worlds so high
like a diamond in the sky

New to the world the child is wonderstruck to see all around it. The distant stars and the sky are beyond his understanding, but soon it discovers the equivalents on the earth as diamond. On one side the heavenly bodies - the sky and the star and on the other the earth and diamond. The poem connects the divine and the human. More than that the child tries to explore the unknown, that later comes back as an eternal query for 'who am I?'
 
To sum up, it seems to me that the nursery rhymes have different layers of understanding, starting from innocence of the child to the higher understanding of the divine. These poems are relevant to every age group.