First - I like the way you think.
The advantages:
- Consistent sizing - no kid getting a corner piece while another kid cries in the corner with their middle-piece-with-some-icing-scrapped-off-bootleg-piece
- Portability - play can continue rather than trying to wrangle kids to sit down and don’t-bring-that-cake-into-the-living-room-young-man
- Cleanliness - Cake has a very high fault rate due to the frosting sticking to everything - kid’s face, TVs, party clowns, presents, parental hair, the bottom of chairs, etc. Milkshakes are hard to spill (but when they spill it is catastrophic)
There are some downsides however:
- Timing - the essential secret ingredient of milkshakes is timing which is one of the reasons behind milkshakes unbelievable power - a cake is good for a few days while a milkshake’s fleeting “viable consumption window” reminds you of the escapist nature of time - always running away and never towards you as you try with all your might to stay in one place. So the milkshake could melt too much or be too thick for the kids.
- Cost - A cake might seem expensive but is cheap in terms of preparation time - you make a phone call and then run by to pick it up the day before or of the party. A milkshake would require a special order and a same-day pickup a few hours or minutes before the party. If you take matters seriously, as those of us who care about things do, the rental of a specialized freezer truck and class C driver’s permit would be required.
- Unconventionalism - While to me putting a candle at the top of a milkshake and singing happy birthday might make for dramatic juxtaposition and symbolism it will probably make the rest of the parents mad and - let’s face it - totally creeped out. The only solution here is to put a cupcake on top of the birthday boy or girl’s milkshake then reinstall the plastic dome to prevent wax from dripping in.