Did you give the early bedtime a go?
Sorry no experience with using incentives/rewards for staying in bed here but I know with a gro clock you do need to be very consistent to see results. So if LO wakes up before lights on/sun up then you need to put them back to bed, remind them it is night time and not to wake you til lights on. A sticker reward may help some children but I think that's up to you. The consistent response is probably what's more important.