While doing housewarming ceremony, what's the significance of the below?
- Bringing a cow and its calf inside the new home
- Boiling milk and then let it spill over
Cow and calf are brought to house and the owner garlands them. According to Hindu philosophy, cow is considered to be the home of 330 million gods and by bringing the cow inside the house, it's believed that we invite/offer all the [demi]gods to our house. Above links also relates "cow" as a symbol of Lakshmi or Shiva.
From the various scriptures, like Surabhi, Nadhini, the cow is often symbolized as KAmadhenu. Dhenu stands for cow and KAm stands for wishes. Bringing cow inside the house thus relates to wish fulfillment.
Milk here signifies the wealth. Due to its fatty (snigdha) nature, it's often categorized as the best SAtvika food. In general, milk is offered to God while pooja. Infants get their initial nourishment via milk.
During housewarming, depending on the family traditions, certain people mix rice and milk and feed to the recipients. While certain people spill the milk (as you asked in question) on to the floor as a coolant, similar to how it's offered to Shivalingam.