"Hey, whose kid are you?" What had been clinging to my leg was a tiny kitten — a fluffy, gray-striped cat.
Literal
"You, where [genitive-の] child [copula-だ]?" Leg [to-に] clinging-came [nominalizer-のは], small kitten was. Gray [genitive-の] striped-pattern [genitive-の] fluffy cat [copula-だ].
A three-part narrative passage: dialogue, revelation, and description. お前 (rough masculine 'you') and どこの子だ ('whose kid are you,' literally 'a child of where') set a gruff but affectionate tone. 纏わりつく (まとわりつく, 'to cling to / to swarm around') appears here in kanji — a vivid verb for persistent physical attachment. ふわふわした ('fluffy') uses the mimetic ふわふわ + した to create an adjective. 灰色の縞模様 ('gray striped pattern') chains two attributive の phrases. The cleft ~のは~だった reveals what was clinging, building narrative suspense.