"We're celebrating our child being with us for a whole year! " "Guess who's the lucky ONE! " "He's the ONE we're having a party for. " "She's turning one, but we don't want to be alone for this birthday party. "
Consider: You are cordially invited to attend the first birthday of [name]. It will be held [date] at [time]. We'll be celebrating at [place]. We sincerely hope that you and your family will be able to come and help us celebrate this special occasion. You can also go less formal.