They wished to take the energy of nature into their body by eating wagashi 

Hina-arare (small rice crackers) is a kind of wagashi (Japanese style sweet) that is offered at the Dolls’ Festival, on March 3rd.
Long ago, the Japanese people had a custom of going to hills, mountains and the seaside with a doll, and to enjoy the spring scenery for the Dolls’ Festival Nippon. Our ancestors used to float dolls out into the ocean on March 3rd.

After their feast, they ate Hishi mochi (rice cake) for dessert. At that time, they ate Hina-arare for dessert with their feast, too.

Reference  http://www.toyosu.co.jp/hinaarare/

Each color has a particular meaning:

The white means “the energy of the earth.”
The red means “the energy of life.”
The green means “the energy of trees.”

They wished to take the energy of nature into their body by eating Hina-arare, and to be able to grow up in good health.