Late to the party, but imo you should look at the Sigma 50mm f/1.4 DG HSM Art with an adapter. Save your cash!

Before locking in a purchase for those backyard shots, I need to know roughly how much distance you usually keep from your kids when taking pictures. Are you standing right next to them, or are you chasing them from across the lawn? Regarding what #2 said about adapting older glass, you have to be slightly careful with autofocus speeds on third-party lenses when shooting fast-moving children. Let us look at a quick comparison of two native options within your budget constraints. First, the Canon RF 85mm F2 Macro IS STM. Pros: incredibly sharp, features built-in stabilization, and sits comfortably under six hundred dollars. Cons: the STM autofocus motor can sometimes hunt a bit in lower light when tracking erratic subjects like running kids. Second, you might want to consider the Canon RF 50mm F1.8 STM. Pros: extremely affordable at under two hundred bucks, very bright aperture for indoor low light, and tiny. Cons: shorter focal length means you have to get much closer, and there is no optical stabilization. TL;DR: If you need reach and stabilization, grab the 85mm. If you want indoors low light on a tighter budget, grab the 50mm. Let me know about your shooting distance so we can narrow it down!

Stop stressing over the massive RF lineup right now because your budget and kids running around call for one absolute no-brainer. Grab the Canon RF 85mm F2 Macro IS STM right away! It costs way under your eight hundred dollar limit, coming in right around six hundred bucks, and it is honestly fantastic for kids and low light. That built-in image stabilization is a lifesaver indoors when the house is dim and the little ones wont stay still. Plus, getting an actual RF glass means no clunky adapter ring to mess with. You get gorgeous background blur that makes portraits pop without selling a kidney. Order it today and you will totally nail those family photos in three weeks!