Hint: A little bit of algebra might help.

Answer: There are four boys and three girls.

If we call the number of boys B, and girls G...

From the first statement (we take one off as Daniel is a boy):

              B - 1 = G

and similarly:

        2 * (G - 1) = B

Substitute G from the first equation into the second equation to give:

  2 x ((B - 1) - 1) = B

    2 x (B - 1) - 2 = B

      2 x B - 2 - 2 = B

          2 x B - 4 = B

                  B = 4

which means that B = 4 and hence G = 3. QED.