Infants do not have strong immune systems. A quick perusal of an academic database yielded three articles within the first 10 hits that caused concern (search terms tea infants):
Stojanović, M. M., Katić, V., & Kuzmanović, J. (2011). Isolation of
Cronobacter sakazakii from different herbal teas. Vojnosanitetski
Pregled: Military Medical & Pharmaceutical Journal Of Serbia &
Montenegro, 68(10), 837-841.
Scientists tested 150 herbal teas and found cronobacter sakazakii in 48 of them (32%). Cronobacter sakazakii is a food-borne pathogen noted for causing serious illness in infants after being ingested in infant formula (Center for Disease Control). The conclusion of the study is that herbal teas should not be fed to infants or people with compromised immune systems.
Ize-Ludlow, D., Ragone, S., Bruck, I. S., Bernstein, J. N., Duchowny,
M., & Garcia Peña, B. M. (2004). Neurotoxicities in Infants Seen With
the Consumption of Star Anise Tea. Pediatrics, 114(5), 653-e656.
The title to this is self-evident.
Bakerink, J. A., & Gospe Jr., S. M. (1996). Multiple organ failure after ingestion of pennyroyal oil from herbal tea in two infants. Pediatrics, 98(5), 944.
The herbal tea in both cases cited in this article was mint.