summer hols and not sleeping

[kbaby]

DD is 4 and has never needed much sleep ie 9-8 but since breaking up from school shes not going to sleep until 10.30pm which is just too late.
What can I do to try and get her to sleep earlier?

shes got blackout blinds but she opens them and even though I put her to bed at 8.30pm when im back downstairs I can hear her turning the light on, opening the blinds, reading books and just playing up there.
Should I just let her get on with it or try and make her lay down and sleep?