They lack energy and the nutrients they need at the most critical time in their physical and cognitive development, the report says.
Only half of children ages six months to 11 months receive any foods from animal sources, which are essential to supply zinc and iron.
Infants and young children have greater nutrient needs than at any other time in life, the report says, but the bodies and brains of millions of young children do not reach their full potential because they are receiving too little food, too late.
France Begin, senior nutrition adviser at UNICEF, says “poor nutrition at such a young age causes irreversible mental and physical damage."
UNICEF data show that:
Less than one-third of children in this age group eat a diverse diet – meaning from four or more food groups daily – causing deficiencies in vitamins and minerals.
The high cost of foods from animal sources makes it difficult for the poorest families to improve their children's diet. In Sub-Saharan Africa and South Asia, only one in six children from the poorest households aged six to 11 months eats a minimally diverse diet, compared to one in three from the richest households.
Almost half of pre-school aged children are anemic.
Young children wait too long for their first bites. One in five babies hasn't been fed any solid foods by the age of 11 months.
Half of children aged six months to two years are not fed the minimum number of meals for their age, increasing their risk of stunting.
Cash or in-kind transfers to vulnerable families, crop diversification programs and fortification of staple foods are key to improving nutrition for young children, according to UNICEF.
"We cannot afford to fail in our fight to improve nutrition for young children," Begin said.