While there are many unknown variables that make this difficult to sort through, I admire the support, love, and thoughtfulness you and your partner are putting into trying to do the right thing for his daughter. The two of you working together to figure this out and be there together for her will mean more than any particular strategy for coping with this situation. That said, I think there are constant transitions a 4-year-old goes through with many steep learning curves. She is likely getting ready for preschool and learning more, and rather than fight the crying for mummy or dealing with “I don’t like crusts,” I’d redirect her. This is a method of acknowledging what she is saying as you help her get oriented to being with you.
“I know you miss mummy and I think she would want you to have a good time while you are here—so lets do this….” Is a sample of this style of intervention.
Alternately, “I don’t like crusts,” can be responded to with something like: “I’m glad you know what you like and what you don’t and are telling me. Let me show you how you can take the crusts off.” These strategies help her to acknowledge her feelings, while redirecting her to the loving and helpful situation you and your partner have for her.
Wishing you patience and peace,
Proof Positive Blog @ PsychCentral