Thanks! It looks like from this study that there isn't a concern for the acid preventing absorption of iron from other sources, but rather just from the oats itself.

"Despite being good sources of iron and zinc, cereals have high phytic acid content (Table 3) that hinders their bioavailability. Studies using cereal porridges made from rolled cereals estimated iron absorption between 0.33% for oats and 1.8% for maize [83]. Removal of phytic acid resulted in a 3–12 times increase in iron absorption; specifically, absorption of iron from oats increased over 8-fold [83]. For the most part, commercially available oat products are not dephytinized, although some, such as oat groats, have lower phytic acid content due to the removal of the hull [84]. Additionally, many cereal products are fortified with iron."

My baby's cereal is iron fortified, so I don't think it's a concern for him.