Were these landscape timbers pressure treated?
If so what are they treated with?
The old CCA PT wood had aresenic in them and the primary concern with those was that children could ingest that aresenic, but it took around 50 years to discover that. The newer PT wood can have ACQ (Alkaline Copper Quat) or CBA (Copper Azole) which, if you ingest that could be harmful. Some research shows that these heavy metals do not migrate very far in the soil and that plants do not generally pick more of these heavy metals than would noramlly be in the soil anyway. However, we do not know for sure what these really do in the garden, so it is your choice whether to use them or not. Some people will tell you to line this wood with plastic but I am of the opinion that plastic will do more harm that the pressure treated wood.

I would take them out right away and use some type of stone. You could wrap the wood in bubble wrap, but the jury is out on plastic material. I do know that in container gardening, a lot of people use bubble wrap to line their pots to keep the soil warm in cool weather.

First, stop panicking :) Yes, some older PT lumber is toxic... in the sense that in some limited conditions, arsenic or other PT compounds may (or are likely to) leach into the soil or groundwater. However, if you aren't licking your timbers, aren't burning them, and aren't running your water supply over/through them, the odds are good that you won't be exposed enough to cause problems.

Toxicity of PT lumber depends on the kind of pressure treatment used. If these are the old type, they were treated with an arsenic solution and you should make an effort to remove/replace them (remembering to dispose of the old ones properly). If it's the newer copper treatment, toxicity isn't really documented as an issue (the reason for the newer copper approach was to make them nontoxic vs the arsenic treated ones), other than when burning them.

As to what you can do: either replace them with stone, or with one of the more-weathering resistant woods, such as redwood, cedar, or cypress, depending on your climate and preferences.

The problem with the old CCA PT wood was that the Arsenic would leach out on to the surface, children (and others) would make contact that Arsenic, get it on their hands, and then potentially ingest that Arsenic.
Arsenic, in very small quantities, is an essential plant nutrient and it is in most every soil already. Adding a bit more from pressure treated wood is most likley not going to be a major problem. But, like lead in paint, children ingesting this poison is a real problem.