One approach would be to allot a proportion P[k] of grant money to each kid
such that the relative allotments are proportional to the relative damages.
That is, with D[k] being the damage by kid k, you'd get an system of
constraints for kids k and j by equations D[k]*P[k] = D[j]*P[j].

Plus that sum(P[i])=1.

I.e., in your example, kid 1 should receive 0.6/0.1=6 times more than kid
3, etc.

The general expression would be: P[k] = 1 / sum( i=1..N : D[k] /D[i] )

It results in the following allotments for your example:
P[1] = 1 / ( 0.1/0.1 + 0.1/0.3 + 0.1/0.6 ) = 67%
P[2] = 1 / ( 0.3/0.1 + 0.3/0.3 + 0.3/0.6 ) = 22%
P[3] = 1 / ( 0.6/0.1 + 0.6/0.3 + 0.6/0.6 ) = 11%

Unfortunately the approach falls to pieces if ever any one kid fails to
do some damage. (As if that could happen )

Another approach is to bring in a concept of "non-damage", such that when
100% damage is done, the N kids did (N-1) non-damage, in the proportions
(1-D[k]). By that approach, the allotment expression would be
P[k] = (1-D[k]))/(N-1)

For your example, the allotments would then be 45%, 35% and 20%.

The second approach deals well with kids not doing damage, but might
be seen as unfair in not preserving the relative damage proportion between
the kids.