Could be snails or caterpillars. For snails you can use a product called Slug-go
which is not toxic to pets or humans.
For caterpillars use Safer Caterpillar Killer, or another product containing the organic ingredient B.t.
Not to gross you our too much, but snails leave curly poop behind, while caterpillars' is more granular.
OR, if the lettuce is full grown, since it's hot it is likely to bolt soon anyway. Pull it all, discard damaged leaves, wash, dry and store wrapped in paper towels in an airtight container. Freshly picked lettuce will last up to 2 weeks this way.