You need bottle-fed lambs to be eating creep when you wean them, usually at about 6-8 weeks although some do it sooner.
With a lamb on grass with mum, you wouldn't be weaning before 12 weeks, mostly of us 16-20 weeks if at all, so no need for creep at this stage.
When/if you do wean them, it is usual to put them on very good grass. Depending on the time of year and what your grass is like, you may decide to give them a little cake at that point, or maybe not until later / autumn - or if your grass is good, and you have good hay too for after Christmas, maybe they won't need cake at all.