Try it another way.

The child is small, mtoto ni mdogo.
Saidi is the child, Saidi ni mtoto. Therefore Saidi is small, Saidi ni mdogo.

The children are small, watoto ni wadogo.
Saidi and Juma are the children, Saidi na Juma ni wadogo. Therefore Saidi and Juma are small. Saidi na Juma ni wadogo.