Can you use royal icing transfers an put on top buttercream cake an cupcakes??

the grease from the bc will soften them -- just depends on what you're doing -- I wouldn't put a 3-d royal icing thing on bc but i've pipe royal onto bc and it was ok

Agree with above. I put royal icing eyes on some cupcakes and it was the biggest mess ever. 

I'm very bad with free hand decorations so my intent was to trace in royal icing and transfer to cake. See picture below

I would either do a buttercream transfer (lots of directions on how to do that on the internet), or use fondant. Just print the shape and cut out around it for the onesie if using fondant. For the rattle, just use a round cookie cutter, or a glass or anything and stamp the shape in the frosting and then trace around it and fill it with buttercream. You can line with a toothpick for the rattle handle if you're worried about that being straight. Same for the booties, either make out of fondant, or use a toothpick to draw what you need and fill in.

whatever works best for you --

i would suggest making  a pattern of the onesie, and after the blue cake icing crusts, trace it onto the cake -- fill it in with the white buttercream so it's a thin covering -- pat down the piping lines with a smooth uber clean kitchen towel right away before the icing crusts over -- patting it down will enlarge it slightly so it will cover all your tracing lines -- practice this first -- then just add the details -- the booties are piped with a round tube -- a shell and a dot, outline with a small tube --  the rattle is a circle with a line for the handle --

once you get the outline of the onesie on there -- you can cut out the different pieces so you can use them as smaller patterns for the accent piping too --

just print the picture of the cake and enlarge the print to the size you want it to be on the cake -- voila, your pattern!

or do a frozen buttercream transfer -- but the little suspenders and the outline of the binding on the onesie is so much cuter on top of it rather than embedded into the transfer kwim