Santa Clause or Santa Claus is OK for a 7 year old as that is a perfect age for believing in Santa Claus, although some kids stop believing in Santa by then or before then, but some kids may continue to believe in Santa Claus past that age.
Most kids stop believing in Santa Claus by the ages of 8 to 10 years old, or roughly around the 3rd grade to 5th grade.
The disbelief in Santa Claus often happens gradually as children begin to notice the logical inconsistencies like travel times and chimneys and also recognize the handwriting on gifts or even hear any doubts from older peers at school.
Although there's no actual single "correct" age that a child has to stop believing in Santa Claus as every child also develops their critical thinking skills at their own pace.
I actually stopped believing in Santa when I turned 5 years old as I could understand that there's no way that he could get down a chimney or make the trip around the world in one night.
But my sister continued to believe in Santa Claus until she was 11 and so it can vary.
The belief in Santa Claus can vary from child to child but the belief in Santa Claus often goes as follows:
Ages 3 to 5, the child often has pure, magical belief in Santa Claus.
Ages 6 to 8, children often have a strong belief in Santa Claus, but the first real questions about the logistics often begin to surface.
And ages 9 to 10, children often start having active questioning, with many kids figuring out the truth on their own about Santa Claus and hear it from classmates.
I personally never told my kids that Santa was real and instead told them the truth from the beginning as I don't like lying to them even though it can seem innocent.
I told my kids from the start that Santa Claus is a magical person that is just make believe and that it can be fun to think of him as being real, but in reality he doesn't exist.