Here’s a site with the song that goes along with the famous poem What Can I give Him by Christina Rosetti, SentForth.net.
This is the stanza that children can memorize:
What can I give him, poor as I am?
If I were a shepherd, I would bring a lamb;
if I were a Wise Man, I would do my part;
yet what I can I give him: give my heart.
Here’s the complete poem from the site AllisonCrowe.com

In the bleak midwinter, frost wind made moan,
earth stood hard as iron, water like a stone;
snow had fallen, snow on snow, snow on snow,
in the bleak midwinter, long ago.
Our God, heaven cannot hold him, nor earth sustain;
heaven and earth shall flee away when he comes to reign.
In the bleak midwinter a stable place sufficed
the Lord God Almighty, Jesus Christ.
Angels and archangels may have gathered there,
cherubim and seraphim thronged the air;
but his mother only, in her maiden bliss,
worshiped the beloved with a kiss.
What can I give him, poor as I am?
If I were a shepherd, I would bring a lamb;
if I were a Wise Man, I would do my part;
yet what I can I give him: give my heart.