I feel that Wolfram Alpha should be able to handle input like:

6, 9, ?, 15, ?

or better still:

6 9 ? 15 ?

and be able to:

• guess the missing terms
• guess the generating formula

The Wolfram language has functions like FindSequenceFunction that can help outside of the Alpha environment; but that's not relevant for primary-school-age users who are entering text into Alpha as a starting point for learning.

Am I missing something? Can Wolfram Alpha parse this sort of input meaningfully?

It can handle this, where there are no missing terms in the middle:

6, 9, 12, 15, ...