Light hearted 
Dd (4yo) thinks that I am. Her birthday is one month before mine. MIL sent me a box of Lush stuff, all very sparkly and colourful, and I foolishly opened it in front of Dd. She is now alternating between begging to 'share' (take) my bath bombs, and insisting that it must be a late present for her. I already gave her one but the rest I quite fancy keeping for me. I ended up suggesting she use her birthday money to buy her own bath bombs, although I should probably be encouraging her to save it. I literally never spend money on pampering myself.
So, YABU: a grown woman does not need a fluorescent glittery bath. Let Dd have them and make sure she saves her money.
YANBU: a grown woman should not be nagged out of her present by her 4 year old. Dd can use her own money if she wants more and let it be a lesson in budgeting.
Also worth a note that usually Dd shares a bath with her 10 month old sister, whom I do not want covered in glitter, so letting Dd have them will generate extra work in terms of bathing them separately.