Here is an example of the Vienna Boys Choir that will give a flavor of what it might have been like for Haydn to sing in a boys choir. This video features the boys singing the famous “The Blue Danube” -- a folk song. The Danube is the second longest river in Europe and flows through Austria and Germany. The following links are all pieces from Haydn.
String Quartet, Op. 73, No. 3 in C Major “Emperor”
(A string quartet is a piece of music written for 2 violins, viola, and cello. It usually has 4 movements, and the 2nd movement is the melody of the famous Austrian national anthem.)
Symphony No. 94 in G Major “Surprise”
There are 4 movements in almost every Classical symphony. This is the famous second movement, and you will definitely be surprised by the loud chord, even if you’re expecting it!
Symphony No. 45 in F# minor “Farewell”
This is the fourth movement of the symphony where the musicians leave the stage one at a time before the end, and these performers are having a good time with Haydn’s humor!
Piano Sonata in A flat Major
“The Heavens Are Telling” from Haydn’s Oratorio “The Creation”
This is just one movement from this beautiful multi-movement work for choir, soloists and orchestra.
Resources for Children
Haydn (Famous Children) by Ann Rachlin
This one might be a bit difficult to find as it is out of print. Try interlibrary loan if your local library doesn't have it.