SOLVED!: Two kids go underground through a tunnel in a city and find a magical world [s]

[Dovewing]

The book starts out with this boy whose mother was very strange. He went to the city and through a sort of tunnel in the central square. It was near a hotdog stand, I think. When they (he met a girl right before they went through the tunnel) arrive underground, they have red Ls on their forheads. The L stands for 'Loser of magic'. Everyone who lives in this underground world has magic.

[Cathy]

The Dragon's Eye and others in the Erec Rex series.

[Dovewing]

I think that's it!