What is happening here?
Grade 2 adds lengths (cm, m), weights (g, kg) and capacities (l), along with measuring with a ruler, estimating and comparing.
What is new is less the arithmetic than the mental image: how heavy is a kilogram? How long is a meter? Without that image, every conversion is just moving numbers around.
What helps at home
Build reference points: a bar of chocolate weighs 100 g, a carton of milk holds 1 l, a door handle sits about 1 m high, a fingernail is roughly 1 cm wide. Anchors like these make estimating possible.
Estimating before measuring is the real exercise: guess first, then measure, then look at how far off the guess was. When measuring with a ruler, watch the zero point: many children line up the edge of the ruler instead of the 0.
What can go wrong
When comparing 1 m 20 cm with 130 cm, children usually do not stumble over the comparison but over the mixed units. So convert to the same unit first and say it out loud: "1 m 20 cm is 120 cm."
And a number without a unit is not an answer. To "How long?", "5" is not right but "5 cm" is. Consistently writing the unit saves a lot of trouble in grades 3 and 4.