I'm not surprised by this because I have seen many people making the same mistake. Anyway based on my relatives' and friends' experience, here is what I suggest:

If your uncle hasn't done "bibaha darta", then he should do it now and put the wedding date after the date of DV application, which would show that he was single at the time of the application. Then fill the forms as a married person, but do not include the kids.

Then come to the US. As soon as your uncle and aunt get green card here ( which will take some months), apply green card for the kids. Then the USCIS will do DNA tests to verify if the kids are indeed your uncle's. Within 2/3 years of application your cousins will be here in the US.

If your uncle includes all the family members in the application now, then his visa application might be rejected, but this is just my thought. 

 sparty is correct , he should make that the marriage was done after the submission of edv form . if he is states that the marraige was done before the edv submission this will falsify the information that he gave during the edv. Regarding the children it is required that they are included during edv submission whether they are with him/her or not. 

why do lots of Nepali file as single? do they think it increases the change of getting lottery?i knew a dv winner came here leaving his pregnant wife back home and said will have to wait till he becomes citizen to bring her over.