This is или These are:Mrs Browen's children. The children's dog. The children's toys. My sister's picture. My parents'

These are Mrs Browen\'s children.

This is the children\'s dog.

These are the children\'s toys.

This is my sister\'s picture.

This is my parents\' guitar.

This is/these are согласовываем с существительным о котором идет речь, если существительное в единственном числе, то ставим this is, если во множественном, то подходит these are.