def required_child_amount_rule(model, cr, ca):
    """
    In order for a child resource to select a child activity, a certain amount of child budget is required.
    A child activity is selected if and only if the provided amount is sufficient.

    :param ConcreteModel model:
    :param int cr: child resource
    :param int ca: child activity
    :return: boolean indicating whether the needed amount is allocated from cr to ca
    """
    return model.CHILD_AMT[cr, ca] == model.req_child_amt[cr, ca] * model.CHILD_ALLOCATED[cr, ca]