Hello David,

You can do it with help of XML by assigning parent model field value to child as default value with the help of context.

Example:

<record id="view_order_form" model="ir.ui.view">
    <field name="name">sale.order.form</field>
    <field name="model">sale.order</field>
    <field name="arch" type="xml">
        <form string="Sales Order" version="7.0">
            <field name="name" />
            <field name="date_finish" />
            <field name="order_line" context="{'default_date_finish' : date_finish}">
                <form string="Sales Order Lines">
                    <field name="product_id" />
                    <field name="date_finish" />
                </form>
            </field>
        </form>
    </field>
</record>

Thanks.