I'd go with either the 'supper' hour, so maybe 5-6pm.
She can help cook or watch toddler/hold baby while you cook.
Sit with DC1 while he/she eats or hold baby while you sit with DC1 or if she's very good, sit with DC1 while holding DC2 and give you a few mins to have a cup of tea!
Help tidy-up after supper (wipe table, sweep floor, stack dishwasher)
or
the 'bedtime' hour, 6pm-7pm
Ideally, she'd hold/rock/settle the baby, while you spend an hour focussed on DC1 (bath,pj's, story, bed).
It depends on your DC's routine though, if the baby is fussy in the evening then that won't work so well.
Or even the 'getting ready for the day hour' so 8am-9am.She could supervise one or both while you have a shower and get ready for the day. Maybe help DC1 get dressed. Get stuff organised, while you get DC ready (put the buggy in the car, make sure nappy is stocked etc.)