How to add fold-up cuffs to toddler pants?

I make my own toddler pants for my 2yo DS. I would like to add contrasting cuffs to them so I can make them longer than they currently need to be. At this age children grow tall so quickly without adding much to their girth.

My pants are a simple two or four piece pattern which has an adjustable waistband. I hand sew the bottom hem so it's invisible. I would like to add a piece of contrast fabric to the bottom of the legs which would be inside the legs like a sort of lining for the bottom few inches, but would make it possible to turn up the cuff and show off the contrast fabric while he is growing into the full length.

Can anyone explain to me how to do this properly? I am concerned about how to attach the top edge of the contrast fabric to the inside of the leg. Would a hand-sewn invisible hem do the trick? Is there a better way?

TIA.

I think the hand sewn hem would be the best way.