ClearAll[functions, minMax]
functions = {# + #2 &, -# &, # + 2 #2 &, -#2 &, 1/(2 # - #2) &};
minMax = MinMax@*Interval;
DynamicModule[{a = {20, 50}, b = {35, 75}},
Dynamic@Grid[{{Grid[{{Labeled[
intSlider[Dynamic@{a[[1]], a[[2]]}, {0, 100, 1}],
Style["a", 16], Left], SpanFromLeft, SpanFromLeft,
SpanFromLeft}, {Labeled[
intSlider[Dynamic@{b[[1]], b[[2]]}, {0, 100, 1}],
Style["b", 16], Left], SpanFromLeft, SpanFromLeft,
SpanFromLeft}}, Dividers -> All, ItemSize -> 10]}, {Dynamic@
Grid[{{"expr", SpanFromLeft, "min (expr)", "max (expr)"} /.