Can I attach a chandelier light fixture to a ceiling fan?
Is it impractical to ask him to attach the chandelier to the fan?

No more so than asking strangers on the internet. Probably far less so since he can get his hands on the fan and the chandelier to address the points below.
Some points to consider:

How much does the chandelier weigh in comparison to the light kit designed for the fan?

Weigh the existing light kit (if there is one), or look up online to find the light kit appropriate for the fan. If the weight isn't listed, contact the manufacturer and ask.
Ceiling fans are already heavy, adding the extra weight of the chandelier could bring the whole thing down on your granddaughter's head. Not good.


Will the chandelier's ceiling mount match the size of the fan kit mount?

It might be possible to make an adapter, but since ceiling fans vibrate a lot when they run, this needs to be a very secure mount.


Are you willing to have an antique chandelier modified to fit the fan's mounting?

It may take drilling additional holes in the chandelier to make it fit to the fan, whether directly or through an intermediary mounting adapter.


How low will the chandelier hang once it's attached to the bottom of the fan?

Your granddaughter may not bump her head on it (yet), but her parents might.
Shiny, dangley things are very attractive to younger kids (up to the age of, oh, say 12 or so) and are likely to be viewed as play things, not respected as antiques.


How fragile is the chandelier?

Ceiling fans vibrate a lot and this could cause damage to the chandelier.


Does the chandelier have a lot of dangling glass bits?

Ceiling fans vibrate a lot and this could cause a lot of annoying noise.


Was the ceiling fan mounted to a properly secured, ceiling-fan rated box in the first place?

As was pointed out by J... in his (now deleted) comment, ceiling fans are often retro-fitted into regular ceiling light fixtures and are somewhat precarious to begin with. The extra weight may just cause the box to part company from its mooring.
While your electrician son is looking things over to assess the practicality of this modification, he should ensure the fan is mounted in a proper box that's properly attached, just as an added security measure to ensure the fan and granddaughter don't meet in an unplanned and unpleasant way.



Since your son is an electrician, he should be fully qualified to make the determination if this is doable electrically (can't imagine that bit would be difficult at all), and would, I'd imagine, be able to manage some sort of mounting adapter as necessary.
You and the granddaughter's parents (no indication if the electrician is her father or uncle) will need to assess the other points to determine if this is the proper time and method for passing a family heirloom on to the next generation.