It's there in Baudhayana Dharma Sutras 2.9.6. The Mantras are as follows:
OM . I quench the Rudras. OM . I quench Rudra’s male attendants. OM . I quench Rudra’s female attendants.
7 OM . I quench Vighna. OM . I quench Vinayaka. OM . I quench Vıra. OM . I quench Sthula. OM . I quench Varada. OM . I quench Hastimukha. OM . I quench Vakratunda. OM . I quench Ekadanta. OM . I quench Lambodara.* OM . I quench Vighna’s male attendants. OM . I quench Vighna’s female attendants.
8 OM . I quench Sanatkumara. OM . I quench Skanda. OM . I quench Indra. OM . I quench Sastı. OM . I quench Sanmukha. OM . I quench Jayanta. OM . I quench Visakha. OM . I quench Maha sena. OM . I quench Subrahmanya. OM . I quench Skanda’s male attendants. OM . I quench Skanda’s female attendants.
9 OM . I quench Aditya. OM . I quench Soma. OM . I quench Angaraka. OM . I quench Budha.
UPDATE:
Updating the answer in reply to @sv's comment below.
Here is the Dharma Sutra in Sanskrit.
The Mantras are actually Tarpana Mantras (used for offering libations of water to various deities). In such Mantras either "Tarpayami" is present or "Tripyantam".
Here, in the text, Tarapayami is present and which they have translated as "I quench".
The Mantras for Ganesha and Skanda (and their allies) are present in pp 28-29 of the Sanskrit PDF.

So, the exact Mantras for Lord Ganesha are like:
Om Vighnam Tarpayami, Om Vakratundam Tarpayami, Om Lambodaram Tarpayami, Om Ekadantam Tarpayami, Om Vighna Parshadischa Tarpayami etc.
and, those for Lord Karthikeya are like:
Om Skandam Tarpayami, Om Mahasenam Tarpayami, Om Skanda Parshadischa
Tarpayami etc.