How to deep clean a second-hand highchair with non-removable covers?

[Goodiebagh]

Kindly gifted a 2nd hand high chair but its really mucky and the covers are not removable! I want to use it, any tips for a deep clean? Was thinking sticking in garden and scrubbing with bicarb....
Money is tight as im 5 months into mat leave and the replacement covers cost more than the buyer paid at a baby used furniture place
TIA

is it a plastic seat? fairy liquid and shower or garden hose

Sounds like a good plan. For a deep scrubby clean I always use an old toothbrush - they get in all the nooks and crannies