ReactJs: What should the PropTypes be for this.props.children?

Try something like this utilizing oneOfType or PropTypes.node

import PropTypes from 'prop-types'

...

static propTypes = {
    children: PropTypes.oneOfType([
        PropTypes.arrayOf(PropTypes.node),
        PropTypes.node
    ]).isRequired
}

or

static propTypes = {
    children: PropTypes.node.isRequired,
}