I'm afraid the term should be "大人のおもちゃ" - you can see that if you check the Google results. It's one of the cases where OT eats a useful Hiragana (seems it's algorithmically programmed to eat all link-word type Hiragana, and 99% of the time they're superfluous, but not here) a bit like it groups singular/plurals and some synomyms together in English.