Kind of in the "not another McDonald's hack" category, but as a dad with young kids, I am enjoying this discovery (thanks to my nephew).
When you order 5 Babycino's from the McDonald's app for $4.25 this fills up the "Buy 5 Get 1 Free" app loyalty reward. Then you can choose any coffee for free (large flat white $5.00, medium $4.50, small $3.90 and most expensive item you can get for free is a large iced mocha for $6.95).
So for $4.25 I get any coffee at all and the kids get 5 Babycino's to throw at other kids - everybody wins.
EDIT: From the comments, Babycino prices vary depending on location, so final price could be slightly higher or lower.
EDIT 2: To be planet friendly as well as kid friendly, if you don't want all 5 Babycino's, please just tell the barista not to make them.
EDIT 3: Looks like it works some places and not others - YMMV!