Is there a chance parentis = parentibus? "Defend the children for their parents."
Cf. queis = quibus
Also : "In the authors of the 10th and 11th cent. the change of the ablative plural -ibus to -is and vice-versa is common." Page 203, note 488: Moriuht: A Norman Latin Poem from the Early Eleventh Century by Garnerius (Rothomagensis.), Warner (of Rouen), Garnier de Rouen, Warner of Rouen, Pontifical Institute of Mediaeval Studies, 1995.
If it can happen for the ablative in an earlier era it may be able to happen for the dative at a later time, especially for scanning purposes in poetry.