Joyful kids put these on to celebrate (4,4)
I believe the answer is:
glad rags
'put these on to celebrate' is the definition.
(glad rags can mean fancy or special clothes)
'joyful kids' is the wordplay.
'joyful' becomes 'glad' (similar in meaning).
'kids' becomes 'rags' (I've seen this before).
'glad'+'rags'='GLAD RAGS'
(Other definitions for glad rags that I've seen before include "best suit" , "Dressy clothes in happy shreds" , "What partygoers showed up in" , "Fancy clothes for laggards" , "Best clothes worn for special occasions" .)