I believe putting pieces in the museum are also a way of determining the final $ value of your dynasty, but the only requirements are they have to be unique and you need 8 of them, so having 9 should be ok. Since you can't mix and match, the childhood drawings, which have very low $ value, would be the only value you can put in for that heir but the quantity of 9 should be ok.
The only reason it's good to get the childhood aspiration done is it allows you to have more items in the museum for that heir. So theoretically you could have 10 items, each of them valued in the hundreds or thousands of simoleons instead of 8.