The experimental difficulties inherent to the solution chemistry of actinoids and lanthanoids have led to the use of a wide variety of models, from the microscopic to the macroscopic scale, in an attempt to represent their solution properties. Molecular dynamics (MD) simulations, with explicit solvents, have been successfully used to describe the structural characteristics, but the limits on the accessible length and time scales do not allow for an equivalent description of the macroscopic properties. In this study, we propose a multiscale approach, based on MD simulation results, to study the thermodynamic and structural properties of a series of lanthanoid-chloride aqueous solutions. An inversion procedure, based on the approximate hypernetted chain (HNC) closure and the Stillinger-Lovett sum rules for ionic liquids, is used to obtain the effective ion-ion potentials from MD-generated radial distribution functions (RDF). Implicit solvent Monte Carlo (MC) simulations are then performed to compute the osmotic coefficients of the salt solutions. This coarse-grained strategy provides accurate effective pair potentials for the lanthanoid salts, derived from an atomic model. The method presented here is an attempt to bridge the gap between MD and the thermodynamic properties of solutions that are experimentally measured.