For composters, how about collecting human urine and diluting it with water - 1:10 - and then using it as a fertiliser? If horse manure works the same way (Nitrogen) why shouldn't urine? I mean, it's easier to collect if you haven't got a horse, and costs nothing. ;)
Alternatively, if the boys are in the garden and need to come in 'for a particular reason', why not just get them to fertilise the compost heap? Easier and quicker and more fun for them, AND no dirty boots all over the house, unless you're in Germany like me, and people have to take their outside shoes off before coming in.