Caminito del rey with 5 yr old

Will you have your own car where you can drop them off and pick them up later?
Perhaps you and your younger child would enjoy El Torcal de Antequera? It's a park with rock formations, child friendly as well.

We did the walk about 3 weeks ago. We took the train from Málaga to El Choro and then a bus to the start of the walk. The bus ride was a series of switchbacks which would be challenging if you have motion sickness. Once you arrive at the beginning of the walk there are areas to hang out, eat and walk trails. Aside from seeing one 10-12 year old, everyone was teenagers and older. The walk takes about 2 1/2 hours and we only were able to get tickets with a guide and group. Be certain your 8yo is excited and able to complete the walk as it is one way only.
We got our tickets online two weeks before and only guided tours were available. It seems very unlikely you could walk up and get tickets as it's very popular so I wouldn't travel there without a ticket.

We did this last May with a small group tour with transportation from Malaga. There did not appear all that much to do at the beginning or end of the trail for the 3 hours or so your wife and 8 yo would be gone. It might make sense for your wife and 8 yo to see if there is a tour with transportation from Seville so that you can more easily find things to do with the 5 yo in the city. They would also then be guaranteed entrance tickets rather than just showing up day of.

Agree that you and your 5 year old will be much happier in Seville. But for your wife and the 8 year, El Caminito del Rey will be an awesome experience! And, yes, it is important to buy tickets in advance.