Just tell them, “we’re having a girl!!”.

You can give them flower seeds that will grow into pink or blue flowers? Or cut a cake... Everybody likes a cake

https://www.nicepricefavors.com/p-8866.htm This link is just an example $1.50 per keychain. Just google "baby party favors"... you're bound to find something for cheap. Party favors are usually designed to be given in huge numbers, so prices are typicslly low. And maybe you'll even find a favor that's better than a keychain