*As Dori said... it won't go on forever. As kids get older.... they tend to not nap.
AND, as Dori said.... if you do this, and it upsets the parents routines/sleep for their child and child ends up being all icky and stressed... at home because of it... they will not like that. I wouldn't either.
The main thing, is that she nap..... ?
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Wow, she's 3... and heavier now not a baby, and is rocked to sleep?
Do you do that in a chair while sitting? Or while carrying her? For me, that would hurt my back....
IS the parents NOT wanting to rock her??? Do THEY want to stop this rocking her????
If not, then well... that is their choice....
BUT.... I would make SURE... that this is what the PARENTS want..... before you then 'change up' their child's sleep/nap routine....
as you said, the child naps well... and needs it....
Next, IF her parents want to stop this habit.... her parents ALSO have to be trying... to transition her.
They probably still rock her to sleep... because, it's always been done that way, thus maybe easier for them, because other methods, will just result in their child getting a hissy fit about sleep.
I would... gradually take yourself away from her (in body contact), in order to transition her.
- Maybe co-sleep with her. Initially. Then when she falls asleep... then you can get up.
- where does she sleep? In a bed or crib still?
- Tell her you will hold her.... only.
- then have her go in her bed.
- you can still be in the room, or sitting in a chair for "company"... while she settles down.... read to her, keep everything calm and dark. Tell her she is a BIG girl.... explain to her....
- make sure she has wind-down time, PRIOR to her actual nap time. Kids this age need to wind-down first, before hitting the hay...
- have her make her bed all comfy, get her stuffed animals in bed with her or whatever 'lovey(s)' she has.... make herself comfy.... my daughter would call it "making her nest....." before bed. Then she would settle in and get into her bed....
- Make another "ritual" of going to sleep..... for her. Something comforting.... or put on white noise etc.
- Or just pat her back.... instead of rocking....
Any transition like this and taking away a former routine/habit.... probably will result in a hissy fit or an upset 3 year old. But you just gotta try it.
Tell the parents.... to get on board too....
What are the parents wanting to do with her? STILL rock her? She's getting older and heavier... and it just is not practical... nor at this age for the child. She can't self-soothe yet.....
Just try different things.... another sleep/nap "ritual" before nap....