just sit the little tike in the middle of the bed and let it play about - bit of muck is just what a baby needs.
Obviously you'll want to move him/her out the way to sow your seeds, but i don't see why it would be a problem
only danger of having a wee one out in the garden is the sun. So lather on the sun cream, hat, wee parasol and let them sit out there with you as you weed, sow, harvest.