Borax is not safe, developmental toxicity means small doses over large periods of time. Given how bad testicular cancer is and that borax especially affects the testes, why risk it? Milk paint seems to form a natural resin using only uncurdled milk and lime. (citation needed)
Wikipedia article on Borax:
Its LD50 (median lethal dose) score is tested at 2.66 g/kg in rats.  This does not mean that it is safe, merely that a significant dose of the chemical is needed to cause severe symptoms or death. The median lethal dose for humans tends to differ for a given compound from that of rats. Simple exposure can cause respiratory and skin irritation. Ingestion may cause gastrointestinal distress including nausea, persistent vomiting, abdominal pain, and diarrhea. Effects on the vascular system and brain include headaches and lethargy, but are less frequent. "In severe poisonings, a beefy red skin rash affecting palms, soles, buttocks and scrotum has been described. With severe poisoning, erythematous and exfoliative rash, unconsciousness, respiratory depression, and renal failure." 
A reassessment of boric acid/borax by the United States Environmental Protection Agency Office of Pesticide Programs found potential developmental toxicity (especially effects on the testes). Boric acid solutions used as an eye wash or on abraded skin are known to be especially toxic to infants, especially after repeated use because of its slow elimination rate.