The body of the handle should not be too thick and not too thin-otherwise it will not be very convenient for the baby to hold it. From souvenir curly pens, too, you need to give up: they cause interest, but to keep them still awkward fingers is absolutely impossible. It is desirable that there was an elastic band / textured engraving on the part of the body where the fingers fall when writing. Then the hand will not slide. Again, the elastic band is softer than plastic or metal — there will be no characteristic calluses at the joint.
Before buying fountain pen I read some reviews like here avalonpens.com/best-fountain-pens/ because I am not expert in such things. For example,I like to wirite snail letters and own stories so I bought the Parker Sonnet Black Lacquer fountain pen which is the best mid-range fountain pen and has affordable price and comparable quality.