>>Regarding the Valley, will we be able to do small walks or mountain bike rides with the kids ?<<
You should be able to do walks with no problem, especially on the paved trails. If it's been raining, there might be mud on the dirt trails. And off-road mountain biking is prohibited inside the national park.
>>And Mariposa ? will it be covered with snow as well ?<<
Do you mean Mariposa Grove of Sequoias, or the town of Mariposa?
In a relatively normal year, only the higher altitudes will be 'covered' with snow by April (which will not include the town of Mariposa or the Valley floor). But, there is always the possibility of a late-spring storm which might close the roads temporarily.
I hope that the tips about Planning a Spring visit are helpful to you.
SFSusan is right (again!) Mariposa Grove Welcome Center (parking) is about 5,000' elevation. I doubt there will be snow at the side of the road, but maybe up higher after you shuttle in and hike up to Wawona Point. There should be snow under trees & in the shade. You might see some red snowplants.
The bike rental at the Yosemite Valley Lodge should have a map for your rental bike route. Definitely stay on the bike path.