Advice needed to dye dark brown hair blue (at the bottom)

I promised my nine year old (she has very dark brown hair) that she can dye her tips (probably two inches or so) blue over the summer holidays. She’ll be getting whatever is coloured, trimmed off before school starts again on the 5th of Feb. I wanted to take her to a hairdresser but I left it far too late and everywhere is booked until well after Christmas. I’d like advice on the bleach and colour and best way to go about it. (I lice ij Australia)