"Joy personified" might cover it ok imho.

I had been thinking about personified. Do you think either of these would do?

Playing children are joy personified.

or

Playing children are personified joy.

Of the two, then Playing children are joy personified. However, if I had to put a succinct caption under the picture (of happily playing children) I would stay with "Joy personified" on its own.

I'm not actually sure it really is for a caption. I just checked, and it may just be a saying without a picture.