Are you planning on these sessions being your DS's only exposure to French? Because then you're talking about getting to native fluency on a total of under 500 hours' exposure to the language, which seems optimistic to me even bearing in mind that native fluency compared to native French-speaking 5yos might not be the same as compared to adults!
Provided you don't regard the effort as wasted if native standard isn't achieved, sounds good to me. You might want to think about how you want to prime the student to react if your DS is unhappy about the French immersion - being left with a caregiver who resolutely refused to speak the language you already understand could be very distressing, so some sensitivity might be needed so that it didn't either distress him or turn into English-speaking babysitting.
