Flow today, 2410: USGS Real-Time Water Data for USGS 09058000 COLORADO RIVER NEAR KREMMLING, CO
I think you'll be OK with a 4 year old, but it requires an entirely different mindset than a group of surly boaters with 60 years of combined experience. Be careful.
The hot springs will not be above water until at least 1800, bigben might be right at 1300, either way don't plan on it.
Plan on 2.5 hours for Pump to Radium, 6 hours for Pump to Rancho, maybe 8 for Pump to State Bridge. These are guidelines, not actual rules, and depend on CFS and headwind, as well as stops.
Pump, Radium, Rancho, and State Bridge are the only access spots. At State Bridge you have two options: river left takeout before the commune of SB, pay the folks to park and take out. Shuttle access is on 131 south of SB. Or you can take out at SB for free, but you have a bit of a boat carry, under the RR tracks, and IMHO it's worth the money to back your vehicle to your boat across the way.
Be safe and have fun.