I see no reason to doubt the veracity of this post. It is very common for children to put coloured streaks in their hair or apply nail varnish during the holidays.
Last Christmas, after I had admired my 2 DGD's streaks, they gave me a set of small pots of the chalky stuff and on Christmas day applied same to my hair until I looked like a Steven Brown Coo. Washed it on Boxing Day and all but the blue disappeared, but I was left with the blue streak until it grew out.
PS, My blue fringe was much admired by my friends - I was even asked which hairdresser did it
Proud Nanna Hope the powder washes out soon. Please shrug this off and don't let this incident become a long running feud with their Dad.