An apple a day might keep the doctor away, but the same can't be said for apple seeds. The seeds contain cyanogenic glycosides, making them mildly poisonous. If you consume enough of the seeds, you could ingest a fatal dose.
But that's a lot of apples. Healthline did the math: You would need to finely chew and eat about 200 apple seeds, or about 20 apple cores, to receive a fatal dose. If you cut up apples for your children or prefer to eat whole apples down to the core, you may want to remove the seeds, just to be safe.