Doped EuFe2As2 is a unique representative of the 122 type family of iron based superconductors due to two magnetic sublattices, the strong coupling between spin, lattice and charge degrees of freedom, and the observed coexistence of ferromagnetism and superconductivity. The change in the Eu magnetic order from A-type antiferromagnetic in the non-superconducting parent compound to ferromagnetic in the superconducting compound upon doping the Fe or As sites, as revealed by our recent neutron diffraction measurements, is believed to be most likely brought by the change in the RKKY interaction, which is mediated by the conduction electrons from the FeAs layers. It will be quite interesting to check how the RKKY interaction is modified and how the Eu magnetic order develops upon doping the Eu-site, as the FeAs layers remain intact against the Eu-site dilution. We propose to perform neutron diffraction measurements at D10 on two Eu1-xKxFe2As2 single crystals (x = 0.3 (underdoped) and 0.5 (optimally-doped)), to clarify the magnetic ground state of Eu in the superconducting region of the phase diagram and to study the role of K doping in effecting the evolution of the Eu magnetic order.