心楽しい is literally their hearts having fun or enjoying. Saying the word heart is like saying "coming from the heart or literally their heart" so there's that "sincerity" connotation rather than just simply saying "楽しい". Another connotation is that it has this peaceful, soothing, soft feeling, when you enjoy or find something pleasant.
Basically, it is something similar to "enjoying" or "pleasant feeling" or "being happy about something." but the connotations that comes with it are a bit hard to translate in english.
This shows an example as to how it is used.
translation: It might be nice/pleasant/delightful to imagine a life without any stress or strain in a world free from worry, but isn't that too much of a dream(wishful thinking) for a lazy person?
translation: Before you love a person, you must love yourself. Embrace yourself, by enjoying/being happy about being yourself, show your abilities, just by being there, you'll make the people around you happy.
another example is this quote by Margaret Mitchell
translation: Eventually, we'll be in a better place someday. A lovely/delightful/pleasant/enjoyable morning will come.
Lastly one of the examples here already has a given translation.
Translation: The pleasantest was to dine luxuriously at some expensive restaurant - O Henry "The Cop and the Anthem"
so depending on the whole context of what you are reading
Could be translated as "And despite being tired, I (sincerely) had fun.(it was enjoyable/delightful)"
Hope this helps and made things clearer.