The first sentence is definitely correct, the person is the subject talking about themselves and secondarily about meeting children. However to me as a native

The first sentence is definitely correct, the person is the subject talking about themselves and secondarily about meeting children. However to me as a native English speaker (from England) the second sounds wrong. It feels like the Swedish kids are the subject of the sentence, and using "whom" jars slightly. My guess is this may be down to the active way their increasing proficiency was described: "were becoming proficient" in the first sentence sounds passive; whereas "already gaining a good level" sounds active, shifting the emphasis and thereby making them the subject of the sentence.