How Should Stools With Softness And Discoloration Be Treated?

Hello. My two year old daughter has been having extremely runny mustard coloured stools for several months. Sometimes she will have soft browner stools in between, but most are so messy they leak out of the nappy between the legs and mainly up the back ( soiling any clothes she has on- that may primarily occur when we lie her down to change her, but it indicates how watery they are).