props.children

Смотрите в документации тут: https://ru.reactjs.org/docs/jsx-in-depth...

Пример из документации:

Например (в стиле хуков):

import React from 'react';
import PropTypes from 'prop-types';
import { withStyles } from '@material-ui/core/styles';


const styles = theme => ({
    formSection: {
        'margin-top': '20px',
    }
});

function FormSection(props) {
    const { classes } = props;

    return (
        <div className={classes.formSection}>
            {props.children} 
        </div>
    );
}

FormSection.propTypes = {
    classes: PropTypes.object.isRequired,
};

export default withStyles(styles)(FormSection);