but I'm wondering if to do medicine you have to have had the dream of a doctor since you were little ? As in, would you have the motivation to do it if it wasn't a lifelong goal ?
No, its not necessary to have wanted to do it forever. Motivation comes and goes and often your biggest passions when you were younger don't last (or, at least, they stand every chance of not lasting) because people change, priorities change - so unless you're totally not open to growth, chances are the things you love and are interested in are going to change. I'm 28 and am considering at least applying for medicine after not having thought it my thing for my whole life.
Also, currently I'm studying nutrition and my goal is to specialise in paediatric dietetics (3 year undergrad, 1.5 year postgrad). From here, would it be possible to go into paediatrics or not ?
Once you have an undergrad, going to medical school (and hence becoming a paediatrician) is possible. If your undergrad is totally unrelated to science them some preliminary study might be required. Med schools generally like you to have a grounding in anatomy, physiology, and chemistry - also often biology and physics as well, depending. The amount of "official" training required depends on the med school. I have no idea what you would cover in a nutrition course so you'll have to take a look at that yourself...