TOTALLY normal with blonde hair, since blonde has several different "base" colors. White blonde hair has no base color (can actually look 'clear' under certain circumstances). Cool blondes tend to look brunette when their hair is wet. Warm blondes run from strawberry to caramel to brunette. LIGHT BROWN haired folks (looks blonde, but isn't) can actually look like they have black hair when wet.
<grin> NO telling what will happen when they get older. Towheads sometimes stay cotton blonde, but they usually get some pigment (anything from warm or cool blonde, red, light brown, brunette, and even black). Warm and cool blondes can stay the same or or darken into brown or brunette OR lighten to a paler blonde.
It's part genetics, part hormonal, and part sun exposure. My mom is a honey blonde in sunny climates (needs to spend at least a year to 2 years for the color to change), I'm auburn with blonde streaks in sunny climates. In THIS overcast climate she's a dark "clear" brunette (almost looks black, but the tone's off), and I'm mousy brown.