In terms of activities, museums and indoor activity centers are good options for indoor programs. Activities indoors could also include museums, sports centers such as swimming pools, or other such spaces. More information on these sites can be found at http://www.the-little-explorer.be/indoor-play-area_54/brussels_1/kids.asp
For activities in Flanders you can visit: http://www.vgc.be/aanbod/vrije-tijd/kinderen-en-jongeren/jeugdwerk
For activities and events happening in Brussels, you can visit https://agenda.brussels/en#.