No Stupid Questions

Yes, to an extent, but I don't think it has to do with grammar as much as pronunciation. Norwegian (bokmål) and danish are almost indistinguishable when written down, but spoken Danish is pronounced very weirdly (a lot of swallowed and mumbled consonants that causes it to sound like the speaker has gotten drunk on their way back from getting a root canal and is currently struggling to eat a hot potato). Despite Norwegian having a massive range of regional dialects, Norwegian kids learn to speak a lot quicker than danish kids. Largely because danish kids just don't understand what they're hearing for longer. The Danes have to subtitle their own TV programmes because they don't really understand each other. It's a fucking mess. Norwegian kids understand Swedish before danish kids understand danish.

Here is a short documentary on the danish language

Ataturk famously switched Turkey to a modified Latin alphabet instead of an Arabic-based one in order to boost literacy rates. Combined with a huge push to educate people on the alphabet it seemed to be successful.

I remember reading somewhere that Danish children on average learn to speak slower than others.

IIRC (and heavily paraphrasing here as this isn't my specialty to say the least) most languages end up having about the same information density because most humans process language at the same speed. Some languages are spoken faster, but have less density in information, where others are slower but have higher information density. In The end it evens out, more or less

From that, without looking at other facts, I'd guess that babies learn it pretty much at the same speed, no?

Since the main question here has already been answered by the Danish/Norwegian post, I would like to address something different. My native language is Spanish, and I completely disagree with you.

English is a lot easier to learn to speak than Spanish. Spanish has everything English has, plus:

• Conjugations (corro, corres, corre, corremos, corréis, corren, corrés, corría, corrías, corríamos, corríais, corrían, corrí, corriste, corrió, corrimos, etc, etc, etc vs run, runs, ran, running)
• Gendered words (La Tienda, Las Tiendas, El Pape, Los Papeles l vs The shop, The shops, The Paper, The Papers)
• Purposefully misgendered words in certain contexts (i.e. Feminine words that use the masculine article in some occasions: El alarma, Los alarmas)
• Particles that change because of sound (Ostras o mejillones/mejillones u ostras : oysters or / mussels/mussels or ousters; insectos y arañas/arañas e insectos : insects and spiders/spiders and insects)
• Extra sounds (hard R as in "Raton")
• Temporary being verb (Ella es rubia/ella está rubia VS she is blonde/she is currently blonde)

The complications in English are later, after you know how to speak and have to learn how to write it, but we're talking babies learning here. Spanish writing is much easier than English because it's very phonetic, but just the conjugations are enough to drive English speakers insane trying to learn them because in English you use constructions to achieve the same effect, e.g. I run: yo corro; I ran: yo corrí; I would run: yo correría; I will run: yo correré; I used to run: yo corría; so that I would run; para que yo corra; so that I could run: para que yo corriera; run!: corre!; don't run!: no corras!. Different verbs would use the same construction in English but may be different sounds for different verbs in Spanish: e.g. I ran, I walked, I had vs Yo corrí, yo caminé yo tuve (and yes, I get that using run is a bad example here since it's irregular, but it's only one of a handful, whereas Spanish has different conjugations for different verbs plus some irregular ones)

English is an easy language, easier than Spanish IMO. If the child lives in a context where the language is spoken all the time doesn't matter when you talk about a first language. 

Why would Spanish be easier than English?
I would judge it to be slightly harder because of gendered words.

Not research, personal experience:

Even after many years of school/high-school in basque, I learnt it at a way slower rate than English, which was just 1 subject.

I didn't speak neither basque nor English outside school. At most, the difference might be that I consumed a little bit of media in English while none in basque. But all subjects except spanish and English were in basque, so that should make up for the difference.

And I don't think it's just a me thing. Since the curriculum has mostly been the same for all those years of school:

Learn how to say a verb.

That's it. Many years of school just to say verbs correctly.

The exams where mostly just fill in the blank exercises, where the blank was a verb.

I still don't know how to say verbs that aren't the simplest ones.

So to your question I'd say yes. Even though neither are my native tongue, I learnt both since I entered school, but learned them at wildly different rates.

I'm no specialist, but I'd say it is all about exposure.

How often and meaningfully they interact with people speaking different languages makes it easier for them to absorb that information. Is grandma the only person that speaks spanish with them when they visit once a year, while everyone else only speak in english? Yeah, it is going to be harder for them to learn spanish