When you talk about a baby, 'eating' nothing but milk or the mashed food pots, it is not rude in the least and the norm in restaurants all over.
When the child becomes old enough to eat normal food, contact the restaurant if you are not sure the restaurant has something suitable.
Many restaurants will do some simple meat and chips, in the kind that kids in the area will be able to get in almost all restaurants in the country.
If you prefer your kids to eat something healthy instead, they might be able to offer smaller portions of 'normal for the restaurant' food or what they give their own children.
Other restaurants will prefer you to bring your own food for the child that they can heat for you.
It is never rude to ask.