Why do climate experts from the UN/IPCC rarely mention Grand Solar Minimum?
The IPCC do mentions solar minimums and maximums, as part of extremely careful treatment they do of the reconstructions and predictions for the changes in solar irradiance. The Assessment Report 5, Working Group 1, Chapter 8, have a whole section (~4 pages) dealing with solar irradiances.

If the IPCC don't mention grand solar maximums/minumums that often, is because the research in the area have shown that solar minimums and maximums have smaller impact in climate than other factors. Therefore, such factors deserve more attention, like Greenhouse gases or land use changes. These factors deserve more attention both because they have a greater impact on climate and because we can do something about them. When it comes to solar activity we can't do much anyway.

The work of the IPCC is to combine the research of the whole scientific community, their conclusions are not based in the result of a single study but in the combination of pretty much all of them. Some studies could be contradictory, so instead of cherry-picking what suits them, they compare and combine all the studies to have the most reliable answers and a good idea of the uncertainties. If predictions of multiple models differ a lot it means uncertainties are large. If they all give the same result, we can be a bit more confident about it.

For example, the figure 8.11 compares the reconstruction of solar irradiance between years 1750 and 2000 from six studies:



You can see how the Dalton minimum shows up around 1810-1820, but the total solar irradiance change is rather small (less than 1 W for over a total of 1360 W).

Here some excerpts from the IPCC reports that are relevant to this question, and where solar minimums are mention and considered in the wider context of climatic forcing (text between square brackets were added by me for clarification):

Page 662:


  Satellite observations of total solar irradiance (TSI) changes from
  1978 to 2011 show that the most recent solar cycle minimum was lower
  than the prior two. This very likely led to a small negative RF [Radiative Forcing] of
  –0.04 (–0.08 to 0.00) W m$^{–2}$ between 1986 and 2008. The best
  estimate of RF due to TSI changes representative
  for the 1750 to 2011 period is 0.05 (to 0.10) W  m$^{–2}$. This is
  substantially smaller than the AR4 estimate due to the addition of the
  latest solar cycle and inconsistencies in how solar RF has been
  estimated in earlier IPCC assessments. There is very low confidence
  concerning future solar forcing estimates, but there is high
  confidence that the TSI RF variations will be much smaller than the
  projected increased forcing due to GHG during the forthcoming decades.
  {8.4.1, Figures 8.10, 8.11}


Page 690 (explicit mentions of grand solar minimums):


  8.4.1.3 Attempts to Estimate Future Centennial Trends of Total Solar Irradiance
  
  Cosmogenic isotope and sunspot data (Rigozo et al., 2001;
  Solanki and Krivova, 2004; Abreu et al., 2008) reveal that currently
  the Sun is in a grand activity maximum [a.k.a. grand solar maximum] that began about 1920 (20th
  century grand maximum). However, SC [solar cycle] 23 showed an activity decline not
  previously seen in the satellite era (McComas et al., 2008; Smith and
  Balogh, 2008; Russell et al., 2010). Most current estimations suggest
  that the forthcoming solar cycles will have lower TSI [Total Solar Irradiation] than those for
  the past 30 years (Abreu et al., 2008; Lockwood et al., 2009; Rigozo
  et al., 2010; Russell et al., 2010). Also there are indications that
  the mean magnetic field in sunspots may be diminishing on decadal
  level. A linear expansion of the current trend may indicate that of
  the order of half the sunspot activity may disappear by about 2015
  (Penn and Livingston, 2006). These studies only suggest that the Sun
  may have left the 20th century grand [solar] maximum and not that it is
  entering another grand [solar] minimum. But other works propose a grand [solar]
  minimum during the 21st century, estimating an RF within a range of
  -0.16 to 0.12 W m$^{–2}$ between this future minimum and the present-day TSI (Jones et al., 2012). However, much more evidence is needed and at
  present there is very low confidence concerning future solar forcing
  estimates.
  
  Nevertheless, even if there is such decrease in the solar
  activity, there is a high confidence that the TSI RF variations will
  be much smaller in magnitude than the projected increased forcing due
  to GHG (see Section 12.3.1).


Summarizing, the IPCC consider solar irradiance variations, the timing of solar minimums and maximums. However, it consider also many other factors that also affect Earth's energy budget. Then, they do predictions based on the combined effect of all these factors. In contrast, some people tend to focus on just one factor (as solar activity) and erroneously assume that it will dominate over all the others.

For the particular case of solar activity, as you can see in the cites above. The IPCC acknowledge the possible occurrence of a solar minimum in the future, but combining all the models they conclude that there is a high confidence that its effects will be much smaller in magnitude than the projected increased forcing due greenhouse gases.

Part of the reason your question might not be well received is because it starts from an assumption that is false: "climate experts from the UN/IPCC never mention Grand Solar Minimum". They do mention it, and if you follow the references in the IPCC you will find plenty of discussion about Dalton, Maunder and older Grand Solar Minimums. And part of that discussion is to estimate the real impact that those events can have in Earth's climate. Then the IPCC get those estimates and figure out how they interplay with the many other factors that conjugate to determine current and future Earth's climate.