Skip the barlow. The 20mm EP should be serviceable if it's their 20mm plossl or kellner. The 10mm likely won't be very comfortable to use, and can be replaced by a 9mm "goldline."
Since everyone will want to look at the planets, you'll want something that get's you into the 120-150x range, which would be a 4 or 5mm. The Agenda Dual ED/AstroTech Paradigm is a good 5mm EP for the price. There's not a ton of inexpensive 4mm choice, but the TMB Planetary clones are ok.
The inherent chromatic aberration in the scope will show up more the higher you push the magnification. So between that affecting the sharpness and contrast of the view as well as what your average atmospheric conditions will allow, if it were me I'd start with the 5mm, judge how well it does and decide from there if it makes sense to go lower on the EP focal length.