These are our favorite shoes for our twin boys (15 months old).  They love wearing them, don't have to struggle with socks, and keeps the rocks out of the shoes in the yard (compared to other more sandal-like shoes).  When they get muddy, I just rinse the off in the sink.  The bottom does have a lot of little grippy bumps, so mud does get caked in there.  I have a brush I use to really clean them.