I have a 3-tiered wedding cake coming up.  The bride is expecting approximately 125 guests with several being children under the age of 10.  I have considered doing a 6, 8 & 12" tier.  Will this be enough cake for that many people if the bride wants to use the top tier for her anniversary?  I'm not a huge fan of the 14" tier because it seems so bulky. 

Should I propose baking a separate tier for the anniversary cake that the bride and groom don't have to cut at all?

You will not have enough w/ those sizes   Use 14,10,6  instead.   Children can eat a lot

Yuck!!!  1/2 my message was cut off    A14,10,6 will be better    Much better to have more than not enough    If you really want to go w/the smaller one you could use a “kitchen cake “ to fill in the servings but I just don’t like using them   

You could also do a 12", double barrel 9" and 6".  But no matter how you get to the number you should 100% have enough cake for each person to get a slice.  It is always better to have too much then not enough.

If cost is a concern of the couple then you could definitely do as kakeladi suggested and supplement the remaining number of servings with a "kitchen cake".

13x9x5 square gets you there too

http://capitalconfectioners.com/cakulator/cakulator.html

but I made an anniversary and a honeymoon cake for my b&gs with scraps from leveling or leftover batter -- either a 5" or a 4" however many layers -- boxed them up in 6" boxes decorated a bit -- and delivered all ready for freezer or fork -- I made it my gift to the couple -- hooked 'em everytime with that one "then that way you can serve your whole tier cake" is what I would say -- brides loved the idea