Abundant U.S. research documents an "immigrant advantage" in children's physical health. This article extends consideration to the United Kingdom, permitting examination of a broader group of immigrants from disparate regions of the world and different socioeconomic backgrounds. Drawing on birth cohort data (ages 0-5) from both countries (n = 4,139 and n = 13,381), the analysis considers whether the children of immigrants have a physical and mental health advantage around the beginning of elementary school, and whether advantage is more pronounced among low-educated populations. Findings indicate that the children of immigrants are not uniformly healthier than those in native-born families. Rather, there is heterogeneity in the immigrant advantage across outcomes, and evidence of both greater advantage and disadvantage among children in low-educated immigrant families. (Contains 4 tables.)