Nothing to worry about, aparently. My understanding of it is that it can sometimes become evident later on (as in your DD's case) because her tummy muscles are starting to firm up. The hernia is caused by a little opening in the bit between the rectus abdominus muscles where they join together called the linea alba, which is where the umbilical cord inserted. I believe most of them clear up by age 1, and certainly by school age. If it doesn't go away by school age then it's not likely to, so they may have to perform very minor surgery to stitch the linea alba.
The only concern is that abdominal contents don't come out the hole. That will likely hurt, and the hernia may change colour. This is an emergency and should be corrected straight away because you don't want the hole to cut off the blood supply to that part of the abdominal contents (e.g. a bit of the intestine). As far as I'm aware, this sort of complication is very rare, so don't stress. Just watch it for changes and I'm sure you'll find it disappears over time no worries
HTH.