Dig out a bed about 2” wider than the timber and fill this with level compacted sand. Place the timbers in the sand so that they do not move. Rebar doesn’t creep out or stick out when done correctly so that’s not a problem but preferences are king and you want what you want. So the sand will keep them generally in place and you may have to move them back into place if they are forced out of place by fought play or intentional movement. They are safe and should remain in place even with children walking on them or jumping on them. I have seen timbers linked together with eye hooks to keep them in line AND in sand and that makes it more difficult for one to get kicked out of place and it’s not unsightly. I have made angle iron fasteners for the bottom of timbers in jobs I have done and pointed the 30cm iron to sink into the ground. I powder coated it for appearances AND rust prevention (since they were underneath and never seen). These prevent the possible creep of a bar pounded down through the timber into the ground…though I have done this MOST and have never had one creep out. Either way I would powder coat or at least smear with tar to keep the rust from eating it away over the years. A job done well is a job done once.