I can't remember what I did with DD1!

When can I put cow's milk on some porridge for DD2 for brekky? Do I wait til after 1 for that or is that only for if it's their only milk? DD2 is breastfed.

I have expressed a few times to give her porridge but I really can't be bothered. Would it be bad for her to have cows milk? I have not given her any so far. The only dairy she has is yoghurt.