In brief: use soft toilet paper; engage the child and make it fun and pleasant; use learning by example; wipe from front to back; about half the kids master wiping by roughly 4 years old.
Note that we did not use the bidet for children - that could be a good idea. I found from personal experience that this worked for us:
Use soft toilet paper (easier on the child). Roll out extra sheets for the child, e.g., 3-4. Crumpling them into a ball, and wiping with that "ball of tissue" is often easier for the child.
Engage the child. Make it fun and pleasant. Have them select toilet paper brand (or wet wipes, if possible). Start by taking turns who wipes the child: the parent or the child, and mark these with letters on a sticky note above the toilet (P C P C ... - use your actual first name initial).
Use learning by example, as @jcmack mentioned. Some suggest using a doll and teach to wipe the doll.
Wipe from front to back (see below).
Wash the child's bottom daily with warm water and gentle soap, as part of the bathtub/shower at bedtime. I do not trust toilet paper to keep the child's bottom clean, any more than I trust paper towels to keep hands clean.
About half the kids master wiping by 4 years old, but YMMV. The range is wide for mastering toilet skills in general, and in the study below there was not enough data to determine the age range for wiping. Note that this is based on only 1 study (see below).
More details and references:
How To Teach Toddlers To Wipe Themselves | Potty Training Concepts: https://www.pottytrainingconcepts.com/How-To-Teach-Toddlers-To-Wipe-Themselves.html
Make sure to show your child that they need to wipe from front to
back. This is extremely important for both boys and girls, since
bacteria can get into areas when wiping the other way and cause
infections. Once they have pulled their own wipe, you want to help
them go through the motion of wiping. If you are using a potty
training doll, make sure that the doll "wipes" also. This is important
when using model potting training, since you are using a model that
you want your child to replicate and copy. So, even a doll needs to be
wiped after a bathroom use.
Schum TR, Kolb TM, McAuliffe TL, Simms MD, Underhill RL, Lewis M. "Sequential acquisition of toilet-training skills: a descriptive study of gender and age differences in normal children.", Pediatrics. 2002 Mar;109(3):E48. https://pediatrics.aappublications.org/content/109/3/e48.long
When to expect results?
For the skill "Wipes poop effectively by self": median age = 48.5
months (girls), 45.1 (boys), see Table 2. [For the purposes of this
question, median age means that half of the children could master this
skill by that age, and half could not. It does not mean that
everyone could do it by that age. - TS]
How wide is the range?
There were not enough children who attained the skill of "wipes poop
effectively" to be able to determine the 75th percentile for girls or
boys. [And from Table 2, the 25th percentile is about 40 months, which
mean that 1 in 4 children could master this skill by 40 months. - TS]
[...] The range of normalcy for the attainment of individual skills
may vary by as much as a year.
How were the kids in this study selected (for comparison with my child)?
A longitudinal survey was conducted of a cohort of children who were
15 to 42 months of age and attending 4 pediatric practices in the
Milwaukee area [...] from 1995 through 1997. [...] For making the
results more generalizable, recruitment sites included 4 pediatric
clinics in the greater Milwaukee metropolitan area (2 inner city
clinics, whose populations are 60% Hispanic and 75% black, and 2
suburban clinics, whose populations are 70%–90% white).
Gwen Dewar, "Potty training problems and solutions: An evidence-based guide", Parenting Science blog. https://www.parentingscience.com/potty-training-problems-prevention.html . A great summary of evidence, with lots of references, such as the reference to the paper by Schum et al.