Have to agree with trueblu and moondog with their observations. That said, we haven't had one of these China private guide threads in a spell, so let's take the opportunity to visit/revisit some fundamentals rather than search for past threads, not just for OP's benefit but for other readers wrestling with the same question:

Off the top of my head, I can think of these general reasons to hire a private guide:
1) Provision of knowledge, insight, and history of the major city sights;
2) Provision of insight and commentary on modern local life;
3) Provision of recommendations for restaurants and shops;
4) Expediting local logistics especially transport and daily sequencing;
5) Ease language barrier and/or assist with interaction with locals.

With respect to China in particular:
1) Very very few Chinese guides can do justice to this dimension since most of them are devoted to "the script" which may have tangential relationship to the historical truth. There's institutionalized blame for that. Most visitors would do just as well to just advance-read a good guidebook and make some internet printouts for the sights they want to see, and take their materials along for consultation. That's the content part of the equation. On the delivery part, many guides are either too short- or too long-winded. The extremes of just pointing out something superficially without explanation of relevance....or on the other end....droning on and on endlessly about boring minutiae. IMO, it's hard to find one that hits the right balance and especially with an adults + kids group. Therefore, not the best reason to hire a guide in China unless on a specialty tour with a Chinese expert along....which doesn't describe the OP's situation.

2) Some Chinese guides can do a good job at this but generally only in response to the visitor's questions. Most guides wouldn't automatically assume a visitor would be interested in modern life, but get a guide with the right personality and one can learn a lot about modern China and have some fun in the process. Finding a guide with the right personality is the key, though. A tangential reason to hire a guide but not primary.

3) Chinese guides tend to do better at restaurant recommendations for local food, not surprisingly, due to pride factor rather than likelihood of getting a kickback. But recommendations for shops must always be taken with a grain of salt, especially for any guide that comes through an agency, since they will have financial motivations for steering you to certain places. When you don't know the lay of the land and proper prices, best to not ask for shopping recommendations from guides. Again, tangential reason to hire a guide since alternative recommendation sources are available via the internet.

4) Guide definitely can be helpful on this, since they know the eccentricities of local systems. But most useful in places not well served by public transport and not walkable, such as rural or small town areas where distances require a vehicle. Little value-added in big cities such as Beijing and Shanghai with extensive subway systems, also in cities with decent taxi availability and if needing only a couple of rides.

5) Ditto with item 4. In major cities with an established foreign tourist presence, a dual-language guide can be useful but not essential. Mostly an "enhancement" if one wants to interact with locals in parks, etc. In lesser cities and smaller and/or less-touristed places, a guide can be invaluable for the non-Mandarin speaker. However, where only language/translation for logistical matters is needed (not true "guiding") the option of cheaply hiring a local university student to accompany you for a small fee, lunch, and English language practice along with their translation services, exists most everywhere.

In summary, maybe the OP would benefit by a private guide in some places, maybe not. It depends on itinerary. I'm not clear if the OP already has one in mind or is looking for suggestions to fill X amount of time. OP, if you come back to this thread, tell us what you have in mind and we might be able to be more definitive on what we think you can do comfortably and independently, where a guide might be helpful, and where a guide + vehicle/driver would be adviseable. Also mention ages of kids. (moving 2 toddlers is a different equation than moving 2 teens!).

The following, though, I think most of us can agree on:
--Definitely discard the join-in group bus tour idea.
--Safety is not going to be an issue whether you are with a guide or without.
--Reliability--in the sense of guides meeting obligations to show up and do what they promised--is variable but generally favorable. Positive recommendations and referrals for private guides, whether independent or agency-provided, are important.
--Tipping in China is usually uncool and guide agreements should be structured to avoid this need (and expectation).