A fair enough point, however it isn't usually the children that pay to see the film: it's their older family members. While it's a rather cynical thought, there's merit to it.
Part of what's awesome about the classic Disney films (not just the ones from the 90s, but the ones our parents and grand-parents grew up watching) is that the side-kicks are usually fun and loved characters by both the children and adults. As a result, the characters have more longevity to them and are generally memorable in a very positive light.
If Olaf is a character that the children love and adore while watching the movie, that's totally fine. As you've pointed out, Disney is firstly aimed at the children. But it's even better in the long run if Olaf ends up being a character that both kids and adults like. Maybe it's just me, but I've seen so many adults wearing t-shirts with various classic Disney side-kicks over the years. Disney makes money off of merchandise, and if they make a character that appeals to both age groups, they're a bit more likely to reel in more income from products related to that character.