I thought I had written about this here previously but I couldn't find it again, so here goes:
Our son #1 heard Spanish from from me, German from my spouse, and English from his part-time British family daycare provider. We had been influenced by two fascinating books by George Saunders. (You might get a copy from a university library or Inter-Library loan.)
In the better of the two books (I forget which is which), Saunders catalogs different approaches to bilingualism in children, and different reasons parents have had for having chosen one approach or another. He also gives practical tips and transcriptions of play-dialogue with his child.
We felt it was important to keep up the parent-couple's common language, English, so our son could be fully integrated into family life, despite my spouse having an intense job and tending toward being somewhat detached, with a prize-winning talent for blocking everything out and daydreaming.
When we left France for the US, as the child was turning 5, we decided to stop supporting French -- other than reading an occasional beloved book in French that we had brought with us in the move.
We (mainly I) had discovered that keeping a language going through the years can be a lot of work. Oh my goodness. Setting up playdates, me struggling to communicate with the French or German playmate's parent, finding appropriate books and song books in the target languages, learning the songs, finding playgroups, making visits to relatives, making supported phone calls to relatives. Also when son #1 was starting to talk, he mixed his three languages like crazy. Since the British child minder only spoke English and French, I kept a list of his current vocabulary in his diaper bag, her his current vocabulary list so she'd have a better shot understanding him.
Arriving in the US we found that in our environs, English took over very easily in daycare or kindergarten, and it took real effort to keep the German and Spanish going -- in the sense of the child responding naturally in the same language he was being spoken to in.
When son #1 was 8, we adopted a newborn. Given the predatory nature of English in our context, we decided not to promote English in the beginning.
By this time my spouse's understanding of Spanish had progressed. Dinnertime was kind of a linguistic free-for-all.
Son #1 spoke what we call "Germish" to his brother, in a 70-30 mixture, favoring Spanish.
My spouse says that talking and singing German with the children has helped them (spouse) be silly, share their sense of humor, and connect more intimately with the boys.
Son #2 attended Spanish-speaking part-time family daycare starting quite early. The provider moved out of state after a couple of years, and the child then started to go to English-speaking part-time family daycare. This child was very outgoing and found the switch low stress.
I did do a lot of planning and orchestrating and providing support for speaking with reasonably correct grammar. But in our interactions, I mostly focused on the meaning and the loving support and the fun.
Now, you: I'm not worried about your child learning Dutch in school or daycare. (Tip: in preparation for school, if your child's Dutch is very limited, you could hire a part-time babysitter for a few months to play with the child in Dutch, to help get over the hump. My son #1 needed that. It helps if the person you hire understands some Italian or Spanish. But the younger the child, the easier it is to pick up a new language.)
My personal opinion is that it's good to limit the number of languages to the number that you and the people close to you have enough time and energy to support consistently over the long term. I've known people from India who had a much easier time with this, because there is often so much daily exposure to someone who speaks language A, someone else language B, and so on on. But wherever we've lived, there's been just one dominant language, so for us it took more effort and planning.
When I was looking for my old post, I came across some interesting Q-A pages:
Should I force the toddler to speak the words and sentences which she is finding difficult to speak?
Can I "read" from English books to my infant, but use words from my native language?
Bottom line, from your description, I see no need for you guys to knock yourselves out with introducing Dutch in the beginning.
I've seen people try to suddenly change what language they've been using as a couple for years -- but the plan generally goes by the wayside as soon as somebody comes down with a bad cold or undergoes a nasty jetlag.
It might not be the end of the world if English were also postponed. In a country like yours, English will be taught well at school.
In short, I doubt you need to introduce Dutch during the parenting leave; whether English is needed early on or not depends on a bunch of factors (see my story, above).
It can be helpful to think about your goals, and some back-up goals just in case the initial goals don't work out. For example, I want my child to respond to me in my language. Or: I want my child to have a close relationship with my monolingual parents. Etc., etc.
I had to let go of one of my goals at one point. When son #2 was diagnosed with Tourette Syndrome, anxiety, OCD and specific phobias at 10, I had to let go of almost all expectations and requirements, including responding to me in Spanish. Life was just too hard for him at that stage.
My adjusted goal has been a success. Son #2 still understands everything said to him in Spanish and German. (Son #1 is now working in Germany for a German company with German-speaking clients; he talks to me in Spanish about 1/3 of the times he calls, and English the rest.)
However many languages you choose, it will be an adventure.