If your child wants to tie dye be smarter than me and do it at the park, not at home. Tie Dying can be a great craft, where you can wear the fruits of your labor for months or years to come. But if you are gonna tie dye with small children around you either you need to keep track of the dye really really well, or do it where there is less to destroy.
And if you can't find the red and the purple you might want to look in your sons bedroom. He just wanted to tie dye his socks. Unfortunately, at four he just isn't all that coordinated. He also dyed a set of sheets (the blanket was black thankfully), his mattress, his brother and the shirt he was wearing at the time, his outfit, the carpet and a whole Thomas the Train set. The Thomas set came clean, the other stuff, not so much.
On the good side my daughter did do a great job on a bunch of socks and shirts.