If "Sunday mornings" is plural, shouldn't the French reflect that as "Les dimanches matins"?
No, I gather in French you use the singular here to mean a repeated usual action.
why can't papa be replaced with mon pere??
"Mon père" means "my father". "Papa" means "dad".
if it is mornings, why is it matin
See Betsy's answer above.
How do you sort out word order? How to distinguish between Le matin dimanche and le dimanche matin?