Please, please ditch the heat lamp! It's overkill and actually works against your chicks' well being. The heating pad system is all they need in the way of heat. In fact, if the rest of the brooder is much cooler, it's even better than keeping the entire thing a uniform temperature. Many of us brood outdoors under the heating pad system and the ambient temp can be quite cool, even in the freezing range. The chicks will become cold hardened much earlier and grow feathers at a much better rate.
Edit: I just noticed you aren't using the heating pad according to the "official" use which involves setting it up on a wire frame enclosed in a soft material to simulate a broody hen for chicks to cuddle under.
The way you have your pad under the bedding does little good. In fact it adds more heat than is necessary. I am recommending you choose either the heat lamp or set up the heating pad system so the chicks can derive full benefit from it. See Blooies's thread in the topic on this forum.
As far as grit goes, you do not put it in their food. Chickens, even baby chicks, will select the exact size and amount of grit their bodies require if you have it available as free choice. I never bother with store bought grit, myself. I brood on sand and the chicks load up the first couple days, and the smug rascals are then all set for any treats that happen to appear in their brooder.
