SOLVED. Children's fantasy with redhead girl protagonist going through garden door to magical world, meets some mute mermaids [s]

[Amanda]

Read around 2010, give or take. Found it in the children's section at Borders (wow remember Borders?) and I remember one of the books showed the main character in a bathing suit underwater with some mermaids. That particular book had a scene where the mermaids told her her hair looked like sunlight. I think the mermaids communicated through sign language. (Main character didn't understand their gestures and another character translated their meaning.) I think the MC was probably around ten years old.
I'm almost certain it was part of a series. I'm not too sure about the plot, but I believe she either moved into a house or went to the house of a neighbor/relative which had a garden. She found a door or gate in the garden, and when she went through it she was in a magical world. There might have been an old lady/eccentric aunt character who owned the house, but I'm not sure. I think for some reason her parents weren't around, so maybe they left her with a relative?

[SBC]

Maybe The Third Wish by Emily Rodda (part of the Fairy Realm series)?

[Amanda]

Yes, that’s it! Thank you!

[SBC]

Cool, glad I could help - I just picked the series up from a secondhand bookshop the other day for my nieces!