I'm thinking it's not possible! 
We got DD a clock for her room & told her she couldn't come in to our room until 7am, but she could play in her room. She's 2y 9m & we've been doing it for nearly 2 weeks now - not working. The problem is that DP gets really angry at being woken, returns her roughly to bed, shouts at her. He is non-negotiable on whether it's something that she's developmentally able to do right now. I feel like it's causing so much friction we should let it go, but he is adamant. All the other mums I know with kids the same age just accept it & get up at 6,30 with them.