Little ones do have accidents,some more than others,and sometimes you need to tell them it's time to go to the toilet,before the accident happens so there's something to be positive and excited about rather than disappointed cos it seems like you're getting nowhere.
If the wet knickers and trousers are part of the problem I'd go without,use them as an incentive to bring back later.It's getting warmer now,no trosers and knickers is much quicker for a busy toddler to deal with in the middle of a game they don't want to interrupt.If you have to go out put pull ups on,but try not to at home as they're so like a nappy they often confuse the child and make the whole thing take much longer than it needs to.
I find bribes go a long way,Smarties the number one choice on our house.Instant reward,right there for each little success.Soon had them wanting to use the potty.
Above all don't let it stress you out,your little girl will see that it worries you.If you need a break you may wel find that it happens just like that when you get back to it.
Have you tried to pin down when she wets - that is, does she get so engrossed in a game she forgets/ can't be bothered to go? My son's 5, and it's only in the last 6 months he's been mature enough to leave a game etc that he's engrossed in to get to the loo on time, though he's understood about toilet etc since we started training him at 2 1/2. I got fed up with reminding him to go, and I think he got very bored too, but it did sink in eventually.
Could it be an attention-grabbing strategy? Does she get more interaction with you when she wets?
If she's wetting all the time, and not just when she forgets, I would say she's not ready. Nurseries normally won't mind (unless they have a strict 'no nappies' rule, and most kids have accidents there - you might find the company of other kids encourages her to emulate their behaviour, as we're seeing with our daughter, wanting to use the 'big toilet' like her brother.
Good luck anyway, and it's only a stage - there'll be another challeng along soon!
My only experience is training my son, who is now 5. We started him at 2 1/2, when he was emotionally ready, and responded to rewards by using the potty regularly. Have you tried a different reward - chocolate buttons might be more appealing than stickers, or a fruit juice drink, or a new crayon, or exciting new pants with princesses on, or something else she will actually want to earn? Also our son continued having accidents and 'dribbles' until he was over 4, as he wasn't physically ready, and no amount of encouragement can help that (though discouragement from the gramps probably isn't helping!). HE also didn't respond well to reminders - he could only 'go' when his body was absolutely ready, and is still no good at 'pre-emptive' tiddles e.g. before a journey.
I would suggest letting her choose a potty she likes, especially for use at grandparents, finding somewhere suitable to put it that's private & comfy, re-educating them that she is not abnormal, and their attitude is a hindrance, trying to work out if there are any other underlying fears (my 2yr old daughter doesn't like the bathroom as she can't open the big, heavy door & fears being trapped in there, for example), encouraging her to use the potty regularly enough and for long enough so she doesn't have so many accidents, plus giving her easy clothing to remove (perhaps gramps aren't helping with clothing & she's getting frustrated/ caught short while she fights tights?). Als lots of general cudles & reassurance from you will always help soothe & relax her, that she's loved regardless. I also received a sample of 'dry like me' - they're like panty pads for kids, so a little dribble is contained to avoid embarassment - might be useful?
It is a difficult & frustrating time, but like everything, it will pass & the next hurdle will come! Keep supporting her, and she'll get there when her body is ready.
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