I like dogs but dogs are messy and smelly and sometimes unpredictable around young children.
It is VERY rude for your mil to presume to bring her dogs.
It is your house. You have a 4 year old and you're about to give birth to a brand new baby. You have ENOUGH things to do without having to worry about how mil's dogs are going to interact with your 4 year old and your newborn. For that reason alone, I would say NO. There is no reason in the world, including appeasing a thoughtless mil, to welcome her dogs into your home at this time. Any "help" she is likely to give will be overrun by having to add the extra duties to your many other duties of watching over the dogs and your children's safety. Over the top!!
Plus the dogs tracking dirt on the floors, smelling up the house so that you can't smell the lucious baby smell, (yes I wrote 3 smells), and them possibly peeing on the sofas and floors, barking, knocking over new baby stuff, waking the baby, accidentally scatching or biting your children, I love dogs but NO NO NO.
Overrule your husband. He'll get over it. You're pregnant, 4 year old, new baby, tons to do, his opinion doesn't count right now, sorry.