Large-amplitude collective dynamics of shape phase transition in the low-lying states of Mg30-36 is investigated by solving the five-dimensional (5D) quadrupole collective Schrodinger equation. The collective masses and potentials of the 5D collective Hamiltonian are microscopically derived with use of the constrained Hartree-Fock-Bogoliubov plus local quasiparticle random phase approximation method. Good agreement with the recent experimental data is obtained for the excited 0(+) states as well as the ground bands. For Mg-30, the shape coexistence picture that the deformed excited 0(+) state coexists with the spherical ground state approximately holds. On the other hand, large-amplitude quadrupole-shaped fluctuations dominate in both the ground and the excited 0(+) states in Mg-32, providing a picture that is different from the interpretation of the "coexisting spherical excited 0(+) state" based on the naive inversion picture of the spherical and deformed configurations.