If you wish to keep the timber finish on a wooden highchair, I suggest usingCabots 1L Clear Gloss Benchtop Timber Finish. It is food safe (once fully cured) as well as non-yellowing and heat resistant. I recommend a minimum of three coats so that it will have a tough finish.
If you are wanting to painting it a different colour, I recommend a hard-wearing finish such asDulux Super EnamelorDulux Aquanamel. These products achieve their non-toxic rating after fully curing, so it's important to give them enough time until the paint is totally dry before using the highchair.
Please make sure to wear personal protection such as gloves, goggles and a paint mask when working. - EricL