Is it this song?

3

If so, I can see (I mean hear) how it would have some strange chords in it. It's very jazzy.

If you want to honor that D# chord, you could just play the note (not the chord) G, because the notes in a D# chord are D#-G-A#. If you pluck the lowest note of the typical G chord a couple times, it will get you past that point.

My own feeling is that the song has too many incidentals in it for a children's song. I believe it was written that way to impress adults, not to please children. If I were doing it, I'd get rid of that funky note. Why not try it both ways and see what works best for your grandson?