That plan is illegal. A B-2 tourist visa is inappropriate for the purpose of coming to get married and stay here permanently.
You need to file an I-129F for a fiance visa, her unmarried child, under age 21, can come with her. You'll have 90 days to get married then after married she and child file the I-485's for adjustment of status. The I-129F supplants the I-130 petitions.
If you marry a nonimmigrant tourist whom you already met a short time ago while she was in the U.S. and want to adjust both her and her kid, you'll have to file two I-130's instead of one I-129F.
Inappropriately using the tourist visa will likely lead to an I-485 being denied due to immigrant intent when she entered on a tourist visa by lying about her intentions and then forcing her to go abroad for consular processing anyway and apply for a waiver for fraud/misrepresentation. They are cracking down on this blatantly fraudulent use of the B-2 visa.
You two met while she was in the U.S,, she went back and wrapped things up to prepare to move to the U.S., this will be obvious to USCIS when she illegally enters on a nonimmigrant visa, marries and files for adjustment. People on this site and others will tell you all the "tricks" that worked for someone at some point, but as time goes by the tricks become obvious BS. USCIS is not full of idiots, they do catch on to these "tricks" and choose NOT to exercise discretion in adjustment. The only "punishment" they can dole out is finding fraud and forcing one to go abroad and go through consular processing AND force them to get a waiver for fraud/misrepresentation (extra form, extra fee and a lots of extra time, effort, expense and aggravation overall).
Take your chances but be forewarned.