What an interesting hobby your Pomeranians have! Seriously, though, it's a bit difficult to answer your question definitively, Pamela, without knowing exactly what you treat your yard with. However, we can tell you that we would not expect significant amounts of most insecticides or herbicides to be present in earthworms, and would therefore not expect a poisoning to occur from an earthworm that was living in a chemically treated lawn.
With that said, we would still recommend discouraging your Pomeranians from ingesting earthworms, as this can lead to stomach upset, and potentially even bacterial-related gastrointestinal problems.