"It is totally childish."
Translation:Es totalmente infantil.
In this case yes. It has to precede. When adverbs modify verbs they most commonly follow the verb they modify, but can precede it for emphasis. But when an adverb modifies an adjective or another adverb (as here where it is modifying the predicate adjective infantil) it always precedes it.
If you are familiar with the acronyms PLACE and DOCTOR for estar and ser respectively you will find that most of the more advanced issues of the use of estar and ser come from the difference between the C in each. Is it a condition or characteristic. In this case he is a childish person (characteristic) Sometime you can argue it both ways even under the same circumstances. In those cases you just have to learn how Spanish views it. But from a post above I saw someone say that if you returned from an animated argument with a friend you might say of that friend, está loco (at least colloquially). So for this case a good way to remember the difference is to think of es loco as he is crazy and está loco as he is ACTING crazy. But more universally that he was in a crazy condition.
As a fluent Spanish speaker who grew in Mexico for all of his life and Spanish is my primary language, I would think of this sentence as: Es totalmente inmaduro. Whereas You are totally childish: Tu eres totalmente inmaduro. Just saying...
I wondered if Spanish made the same distinction between "childish" and "childlike" as we do in English, since "childish" is usually a bit derogatory, but "childlike" is not. I tried looking up the translation for both and both gave me "infantil", but "childlike" also gave me "ingenuo" and "de niño". But on a bit further investigation "ingenuo" seems to be mean something closer to "naive", whereas "de niño" seems a lot closer to the English "childlike". The subtleties of languages get fascinating, but also a bit challenging in the sense of how will I ever learn and remember all of this. I guess that's the difference between stumbling along and fluency.
Ribeiro and wolofka: eso = that; esto = this. You are introducing something else into the sentence. What do you mean "the one-to-one word translation rule"??? What rule is that? It sounds a recipe for disaster! For example, it seems to have led you, wolofka, to forget that the one word "es" can translate as the two words "it is" (and in fact does if no other subject there or no context to assume it means "he is" or "she is") and I think both of you inserted the unnecessary eso/est o becaus eyou were trying to have an explicit subject.
Lo is the direct object pronoun, not the subject pronoun, although there are a couple of idiomatic expressions where it appears more as if it were. Spanish does not have a pronoun for this amorphous "it", and will always just use the third person singular congratulation without a pronoun. Spanish deals differently with these amorphous ideas. Remember it has a special word just to say "there is/there are, and a different form for this and that when the referent does not tie back to a concrete noun. But for this situation it simply takes advantage of the ability to drop the pronoun totally.
Read Lynette response. This would be more like a response though. If someone were to ask you is it chidldish? You could answer Lo es...
In La casa blanca, the adjective blanca comes after the noun it modifies which is Casa. In this sentence infantil is a predicate adjective modifying the subject It and totalmente is an adverb. Adverbs which modify verbs have some options in syntax, but adverbs modifying adjectives ALWAYS precede the adjective it modifies. There are cases where adjectives also precede the noun, but they obviously don't apply here.