The Child was sick and spread that in Serthica, correct. But the one whose magic was everywhere, who revived the half of Serthica that was effectively undead and who gave the Child his "dollhouse" was the White Wanderer. So any of the magic linked to the sickness was the Child's, but the magic holding up Serthica was the White Wanderer's!

The Child's sickness came with him from the volcano of Ke-Waihu :(