My experience with my own nails suggests that longer nails seem
to stay cleaner. I think this is because when you scratch something
with long nails, the bits of stuff you're scratching end up sitting
near the end of the nail and then falling off, but with short nails
the bits of stuff would be pushed into the tight space between
nail and finger when you scratch. I'm not sure if that applies
to kids digging in dirt or not.

If you get some interesting bath toys (cups, washcloths, watermills etc)
and play with them while the child is having a bath, he might
enjoy hour-long baths. Reading stories aloud while the child is in
the bath can work too (as long as you watch the child for
safety while you read). Some children like to stand at the
kitchen sink and play with dishes, coffee percolators and things.
These activities can help get fingernails clean.

If you wash the hands just before cleaning the nails, or
even better soak the hands for a while in the bath or
wherever, I find it's easier to scrape the dirt out because
it's soft.

To clean the nails without hurting, find something not too
sharp and not too fat. If it's too fat, it hurts because
it pulls the nail away from the finger -- especially if
you're trying to clean short nails. One thing you might try
is a piece of paper folded once or twice. You use a corner
where it's folded, and slide it in a direction such that
the dirt tends to go between the folds. You may need to
keep refolding a new corner for each nail. The right object
for a 2-year-old may be thinner than the right object to
clean an adult's nails. You need to be careful not to
poke too hard and hurt.

You can make cleaning nails fun. You can tell the child
stories while the nails are being cleaned. You can talk
to the dirt, and pretend the dirt is talking: "I'm sorry,
dirt, you can't stay there. Fred wants to have nice
clean nails. ... Ha, ha, we're in this nice safe
fingernail. Nobody can get us here. Oh, no -- what's that?
Aaaahhhh!" When you talk to the dirt you can talk as
if it's your child who wants the dirt to go away. A child
will often take on an attitude which seems to be assumed
or expected.

I wonder whether playing with slime would clean the
fingernails. (It would get the slime dirty, though.
Oh, well, it was an idea.)
--
Cathy