For future reference I was able to book via the Android app for 1 adult + 1 child (non-infant) without the warning popping up, and we had a great mid-afternoon DXB-CDG a few days later. As A4K suggested I could indeed have booked for 2 adults with correct dates of birth via emirates.com but unsure how much keyboard-tapping would have resulted at the airport I went the app route.