Can you work out the theme?

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Across
  1. 1. ...califragilisticexpialidocious
  2. 3. Flounder and Sebastien spend their time with this little one
  3. 4. This band had smash hits such as 'Barbie Girl' and 'Doctor Jones'
  4. 7. The New York and Paris undergrounds are both called this
Down
  1. 1. People who are scared of this are arachnophobes
  2. 2. Use this to hit cricket balls
  3. 3. This 'byte' is larger than a kilobyte but smaller than a gigabyte.
  4. 5. Most commonly found with Declan Donnelly
  5. 6. This element has the chemical symbol 'Fe'