The answer “Because it contains the substance chlorophyll, which is green”. But it does not have the necessary knowledge. The fact that chlorophyll absorbs everything except green is the definition of green.
Chlorophyll (from the Greek ??????, “green” and ??????, “leaf”) is a green pigment that determines the coloration of plant chloroplasts in green. With his participation, the process of photosynthesis is carried out. According to the chemical structure of chlorophyll – magnesium complexes of various tetrapyrroles. Chlorophylls have a porphyrin structure and are structurally similar to the heme.

So, the grass is green, because it contains chlorophyll. Chlorophyll is needed to provide the process of photosynthesis and convert carbon dioxide to oxygen, while obtaining energy for the life of the plant. And it is with the process of photosynthesis that the green color of chlorophyll is associated.
White light is a mixture of all colors of the visible part of the spectrum. Each color has its own wavelength of light. Energy is inversely proportional to the wavelength: the longer the length, the less energy. This can explain why chlorophyll absorbs blue colors, because they have the greatest energy.

The absorption of red colors depends on another characteristic of light radiation – the number of photons per unit of energy. Chlorophylls are activated not by energy, but by photons. That is, the more photons are more, the more active the photosynthetic reaction takes place. The number of photons, on the contrary, increases with increasing wavelength.

Combining the energy and the number of photons, we get 5 different areas:

1. Photon is so small that even a lot of energy in each does not allow the photosynthetic reaction to proceed;
2. The number of photons increases, and now they are sufficient for the reaction, in spite of the fact that the energy has decreased slightly;
3. The energy is less, and the photons are not enough, and the reaction is again under threat;
4. There are many photons, they continuously bombard the sheet and even their small energy does not interfere with the reaction: it flows even better than in the blue region of the spectrum;
5. Photons practically lose their energy and their enormous amount does not help anymore.

Depends on your imagination. For example: The grass is green, because at this color it can receive more heat from the sun and grow better.

This answer does not affect chlorophyll-photosynthesizing things at all. But in my opinion, it is quite acceptable. First, he is truthful. Secondly, it explains the reason, and not the property of some kind of abstract chlorophylls. Yes, this answer is suspiciously similar to option # 2. But it follows from your knowledge, which you simplify for the child’s mind. And not from your ignorance, which you hide, so that the child does not think that you …