When it's being held in a posh hotel?

Our DGodson will be 1 next Sunday and we've been invited to his birthday party. His parents had their reception at this hotel, had the after party from DGodson's christening there and now this! His first birthday... 
When DS turned one we just had both sets of GPs round for a cup of tea and a cupcake. This is slightly out of my comfort zone 
I was planning on just wearing smart jeans and a nice top with some heels. Now really not sure that's going to cut the mustard 
What do you reckon? 
PS. DH and I are taking bets over how they celebrate toilet training him...first day at school...first grazed knee...