The material that is used to absorb the milk is a Super Absorbent Polymer (SAP).  It is similar to what is in baby diapers, in that it starts as a powder and once the milk hits it, it turns into a gel.  The SAP is non-toxic, however, it of course is not meant for ingestion.  If you do think there is a possibility that your baby did in fact ingest some of the polymer, contact your doctor and/or your local Poison Control Centre.