Yeah, it’s entirely possible for a child of two brown-haired parents to have blonde or even red hair.

For one thing, blonde hair tends to get darker as you get older, and so a lot of people who were born with blonde hair can end up with dirty blonde, light brown or even dark brown hair. This can mask the fact that they technically have the genetics for blonde hair.

Another factor is that blonde hair is a recessive gene, which (if I remember my genetics correctly) means that not everyone who has the gene will actually gets blonde hair because they need two copies of it instead of one; something which can get further confused by whether you’re born with XX, XY, or some other combination of chromosomes (I think, if I remember correctly, that some genes are more likely to express themselves with XY chromosomes or vice versa, not sure if that’s true for hair color). So you can have a family with brown hair, but if the genetics line up just right, you can get a blonde-haired kid (if I understand correctly, this is also true for red hair).