DS had serial casts for toe walking he wore them for about a month at a time with the casts changed weekly with a gradual stretch each time. He would then have a month off and then be back in for a month. In his month out of casts he also had night splints, sort of plastic L-shapes with Velcro to keep his feet at 90 degrees at night.
It did help to stretch his tendons but didn't break his toe walking habit at all. We all made a massive effort to say 'heel-toe' every time he walked anywhere and would put a hand on his shoulder to remind him.
To be honest DS was not bothered at all by the casts, I thought we would have all sorts of dramas but he could not have given less of a shit. He had them between the ages of 3 and 4 and didn't seem to have any pain associated with it.
DS is hyper mobile and our consultant thought that toe walking helped him to stabilise his hips knees and ankles. We also had some X-rays done as DD1 had developmental displaysia of the hip and some late diagnosed DDH sufferers toe walk to make up for a leg length discrepancy. To be honest DD1 was a late diagnosis and before that we had been overlooked as fussy first time parents (DS is our oldest) and as soon as we had a diagnosis for DD1 DS was X-rayed within a week and we had consultants, physiotherapists falling over us!
One thing we did was 'buddy up' with another DC who was a little ahead of us treatment wise. This meant that when my DS went to have his first set of casts put on we met up with a similar aged girl who was having her first set cut off. This helped to see what the process was like the plaster technician said some DCs freak out about the noise and vibration of the cutting tool thing. My DS giggled his head off and said it really tickled. We kept his last set of casts as a reminder to heel-toe!
He is 7 now and still mostly toe walks but will walk flat if nagged, so the casting process didn't break the habit but did increase the functional use of his ankles.
Good luck and if you want to PM me feel free. (We are now experts at the bin bag wrapped bath and how to sleep with hot feet.)